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SPECIAL REPORT

STATE OF TECHNOLOGY 2017

TEMPERATURE AND PRESSURE MEASUREMENT

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State of Technology 2017: Temperature and Pressure Measurement 3

Taming temperature and pressure 5

Secrets to good vessel temperature and pH control 11

Living on the edge with surge control 15

The deal breakers 19

Resource guide: Temperature and pressure don’t stress 23

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TABLE OF CONTENTS

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Taming temperature and pressureInnovations in design, control and communications are making life easier in heating, cooling and pressure measurement applications.

by Jim Montague

Temperamental people are sometimes described as “blowing hot and cold,” and

many process applications do the same. Handling and optimizing the changeability

of temperature and pressure—if not outright volatility—is one of the cornerstone

skills of the process control and automation field. However, along with their rookie coun-

terparts, even longtime experts can appreciate many of today’s improved solutions for

smoothing temperature and pressure fluctuations, along with supportive data processing

and networking tools that allow wider access to information for better decisions.

“There are more divergent temperature and pressure applications these days, but users still

want solid measurements that are easy to work with and don’t cost too much, especially

as they add more points, gather more values, and improve communications,” says Keith

Riley, national product manager for temperature and pressure at Endress+Hauser (www.

us.endress.com). “There’s less resources and expertise to go around, so the goal isn’t so

much collecting more or different measurements, but more about making life easier for

existing processes.”

UNIFIED COOLING CONTROL For instance, South Florida Distillers (www.southfloridadistillers.com) in Fort Lauderdale,

Fla. recently helped 26° Brewing (www.26brewing.com) in Pompano Beach, Fla., imple-

ment an integrated, cold-side automation system for its 30-barrel brewery, including



KEEPING SUDS COMFY

Figure 1: Conical, jacketed, 30-barrel tanks are cooled during the fermentation process at 26° Brewing in Pompano Beach, Fla., by three glycol-filled bands controlled by AutomationDirect’s Do-more PLC and C-more touchscreen HMI, which also control a brite tank and cold liquor tank downstream. Source: South Florida Distillers and 26° Brewing

controls for precise temperature stabili-

zation of seven tanks, expandable to 16

tanks. The controls include AutomationDi-

rect’s (www.automationdirect.com) Do-

more PLC and C-more touchscreen HMI

that perform process monitoring, process

control, data acquisition and data logging

for five conical, 930-gallon, stainless-steel,

glycol-jacketed fermentation tanks, which

are accompanied downstream by a brite

tank for quick cooling and a cold-liquor

tank (Figure 1).

“Though individual PID temperature con-

trollers could have been used for each of

the seven tanks, one Do-more controller

was a better solution and less expensive,”

says Avi Aisenberg, CEO at South Florida

Distillers. “The added value of the PLC is its

remote viewing, process control and ease of

training new users. This design also required

less electrical work and will be less costly to

expand.”

The temperature of each fermentation tank

is controlled by a PID control algorithm

running in the PLC, which includes two

multipoint AC output modules to control 19

solenoid-actuated water valves. Seven re-

sistance temperature detector (RTD) sen-

sors are connected to PLC input modules

to measure tank temperature using clean-

in-place RTD probes. For each tank, a PID

loop uses its tank’s RTD sensor as the pro-

cess variable input, and controls three ball

valves via the PID controller output. These



valves control flow of a glycol/water solu-

tion in each tank jacket. Each fermentation

tank has three cooling zones, with cool-

ing solution flow controlled by one valve

per zone. The brite and cold liquor tanks

each have two cooling zones and valves. A

ramp/soak pattern can be programmed to

last for days or weeks based on the beer

being fermented.

The automation system’s HMI has a cus-

tom-designed user interface that mimics

product process flow. The controller and

HMI are networked by an Ethernet switch

and wireless access point, which provides

network connections for local and remote

access to the C-more touchscreen via

iPad, iPhone and Android apps running on

smartphone or tablet PCs. “This system

adds makes interaction with the automa-

tion system more user-friendly and easier

to setup than with multiple temperature

controllers,” adds Aisenberg. “The au-

tomation system provides data logging

locally at the PLC, and remotely through

the Ethernet switch, and a free app lets

mobile devices remotely access the HMI.

Once remote access is enabled at the HMI

and the app is installed, duplicate screens

from the HMI can be viewed and con-

trolled remotely from the mobile device.

All process data is periodically emailed to

predefined users or when there’s a high-

or low-temperature alarm, deviation alarm

or other condition.”

TACKLING EXTREME SETTINGS, SAFELYNot stopping at simplifying and combin-

ing control designs, many temperature and

pressure solutions are also acquiring ca-

pabilities so they can serve in increasingly

remote and harsh settings. To help users

test cement used in oil drilling, for example,

Fann Instrument Co. (www.fann.com) in

Houston, Tex., has developed a standalone

ultrasonic cement analyzer (UCA) to house

its pressure and temperature sources, as

well as its computer to control its sources

and store recorded data. UCAs are used in

field labs and drill sites to test cement slurry

samples under simulated pressures and

temperatures to determine initial sample

curing rates. As a subsidiary of Halliburton

(www.halliburton.com), Fann’s traditional

UCA used autoclaves connected to a com-

mon pressure source, while cement curing

rate data is read back to a separate central

computer. After evaluating several options,

Fann selected National Instruments’ (www.

ni.com) CompactDAQ, which integrates

connectivity and signal conditioning into a

modular I/O device.

“Because of NI CompactDAQ’s modularity,

adding functions to our UCA for a custom

machine is as simple as plugging in a new

module,” says Rick Bradshaw, formerly of

Halliburton. “Our new UCA uses an embed-

ded industrial PC, so NI CompactDAQ’s

standard USB connectivity eliminated the



need for external controllers or interface

cards. Also, by using National Instruments’

LabView to create our software, we re-

duced our development time.

“Now, we can control the temperature,

pressure and ramp profiles, and perform

other tasks not possible on individual au-

toclaves. We can also take this standalone

UCA into the field, directly to sites, while

it would have been difficult to transport

the whole system before. We’ve also

potentially saved weeks on designing and

building new hardware, and gained the

flexibility to meet more customer-specific

functions by adapting to a wider range of

I/O requirements.”

Scott Nelson, VP and GM of Rosemount

Pressure Products at Emerson Process

Management (www.emersonprocess.com),

reports, “As our customers push the limits

of their operations, we’re being asked for

more solutions that can withstand ex-

treme environments and severe chemical

conditions. As a result, six months ago,

we introduced our Rosemount 3051S High

Static DP pressure transmitter that can

accurately measure differential pressure

from 5 in.H2O to 150 psi on top of 15,000

psi line pressure. It’s available with wired

or wireless communications, and measures

differential pressure (dP) and process

temperature all in one.”

To enhance personnel safety in these chal-

lenging applications, Nelson adds that

Emerson also recently introduced its Rose-

mount Wireless Pressure Gauge (WPG). Un-

like traditional pressure gauges that include

a mechanical Bordon tube, WPG is based

on Rosemount sensor technology that can

handle 11,000-psi burst pressure, provide

self-monitoring diagnostics and expand

insight with WirelessHART. “We weren’t in

the pressure gauge industry before, but it’s

a need for our customers, so we’re bringing

our pressure transmitter technology into

pressure gauge applications.”

BETTER TOOLS + NETWORKS = BETTER DATALikewise, to enhance the safety of its

boiler control system by generating re-

mote updates, McKee Foods Corp. (www.

mckeefoods.com) in Collegedale, Tenn.,

has implemented eWon’s (https://ewon.

biz) 2001 PSTN router and 4001 GSM/GPRS

modem to notify maintenance personnel of

current alarm conditions and let them shut

down the boiler remotely, using land lines

or cellular phones. This solution allowed

the baker of Little Debbie snack cakes and

other bakery products to comply with state

regulations requiring any boiler systems

to be monitored during operation by plant

personnel, but now they no longer have to

be onsite 24/7.

Consequently, the eWon rounter and

modem communicate with the bakery’s

Allen-Bradley CompactLogix controller via



EtherNet/IP networking to monitor alarms,

including loss of communications, high-

steam pressure and others. Connections

between McKee’s control system and its

operators are enabled by eWon’s Talk2M

server, which creates a network tunnel and

secure virtual private network (VPN) be-

tween sender and receiver.

When a critical alarm is detected, an SMS

text message is sent to a preset list of

cellphones and pagers, allowing opera-

tors to respond to the eWon units via

cellphone or landline, and shut down the

malfunctioning boiler. Staff can then go

to the boiler, and begin diagnostics us-

ing eWon’s data, including alarm history,

events and historical datalogs to deter-

mine the failure source.

Similarly, because temperature read-

ings are critical to the life sciences and

their instruments need frequent calibra-

tion, Riley adds Endress+Hauser recently

launched its iTherm QuickNeck thermom-

eter extension neck with 1/4-turn quick

release and IP69K protection. “Even aided

by mobile devices, transmitters have to

be unwired, opened, checked and wired,

which takes about 30 minutes,” says Riley.

“QuickNeck’s 1/4-turn gets the transmitter

head and probe out of the thermowell and

back in 10-15 minutes, which saves time

and reduces the chance of an alarm, and

lets the RTD continue measuring. Also,

while a regular RTD can only handle 3-4-G

vibrations, we have a new thin-film RTD

that can withstand 60-G vibrations, so

it doesn’t need to be replaced as often.”

Thin-film RTDs also can provide 2-3-sec-

ond response times, which are compa-

rable to a thermocouple, rather than the

traditional RTD response time of 12-15

seconds.

Unlike its foray into mechanically aided

safety with the WPG, Emerson’s Nelson ex-

plains that many temperature and pressure

components are becoming more electronic

and less mechanical because they’re more

efficient, cost effective, and save time for

users. For example, he says Emerson’s new

Rosemount 3051S Electronic Remote Sen-

sors (ERS) system eliminates mechanical

impulse lines and the troubleshooting and

maintenance they often require. Users also

appreciate that ERS only takes one person

less than two hours to install, while tradi-

tional mechanical devices can take two

people six to eight hours to install.

“Previously, dP transmitters were con-

nected to tanks with a mechanical system,

which allowed outside ambient environ-

mental changes to create instrument er-

ror,” says Nelson. “Now, we put two pres-

sure sensors on a tank that are connected

electronically, which eliminates prior

sources of errors and maintenance, and are

five to 10 times more accurate.”

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Secrets to good vessel temperature and pH control

The key aspects of good vessel temperature and pH control are often not recognized.

Here we provide some simple guidance and overview of the benefits of a particular

strategy and implementation guidelines that should be commonly used. A fundamen-

tal understanding is provided that may also be beneficial for other primary control loops.

In case you are busy let’s cut to the chase. Let’s start with pH. The cascade control of vessel pH

to inline pH provides a simple well known open loop gain for the vessel pH loop (primary loop).

The effect of the nonlinear pH titration curve is isolated from the vessel pH loop and dealt with

by the fast inline pH loop (secondary loop).The open loop gain (also commonly known as the

process gain) for the vessel loop is simply dependent upon the scales of the vessel and inline

pH loops. If the pH loops have the same scales, the open loop gain is simply one.

The well-known open loop gain makes the tuning of the much slower vessel pH loop much

easier. The biggest benefit is that you can much more easily avoid having a vessel PID gain

that is too high or too low. The less recognized problem of too low a vessel PID gain stems

from the vessel loop response being near integrating and true integrating for continuous

and batch operations, respectively. A near or true integrating response will cause incredibly

slow oscillations (e.g., period that is 40 times dead time) if the PID gain is too low. In some

ways violation of the low gain limit is worse that violation of the high gain limit because the

oscillations are much larger besides being much slower, which means they are not as effec-



tively smoothed out by downstream vol-

umes. They are also continuative.

The minimum PID gain is inversely propor-

tional to the product of the open loop in-

tegrating process gain and PID reset time.

You don’t want this to be dependent upon

the slope of the titration curve that varies

enormously with pH and feed concentrations

or how well you tuned the reset time. Most

reset times are set way too small since people

tend to use too much integral action and not

enough proportional action on vessels or

columns (any volume with mixing either due

to turbulence or agitation). Often the reset

time needs to be increased by two orders

of magnitude. By simplifying the open loop

gain to a number that is constant and easy

to calculate, the PID gain can be more readily

set to be above the minimum besides below

the maximum. The reset time is simply a fac-

tor of the vessel dead time. For an arrest time

(lambda) of two deadtimes in lambda tuning

for integrating processes, this corresponds to

a reset time of 5 deadtimes. Note that smaller

arrest times can be used by making the pro-

cess gain linear and relatively constant by the

recommended cascade control.

For a well-mixed vessel, the process time

constant is approximately the residence time

that is simply the vessel liquid mass divided

by the process input mass flow. For inline pH

loops on the influent, this is the influent feed

flow. For inline pH loops on a recirculation

line, this input flow is basically the recircula-

tion flow since the recirculation flow is typi-

cally much greater than the feed flow. The

near-integrating process gain is the open

loop gain (dependent upon primary and sec-

ondary scales) divided by the process time

constant (dependent upon residence time).

If the pH loops have the same scale, the in-

tegrating process gain is simply the process

input mass flow divided by the liquid mass

in the vessel. This calculation can be done

on a volumetric basis. You just need to have

consistent flow and liquid inventory units.

The cascade control of vessel temperature

to inline feed temperature or recirculation

temperature, the computations are similarly

simple. If the secondary loop is jacket inlet

temperature with constant jacket recircula-

tion flow, the open loop gain is again simply

dependent upon primary and secondary

loop temperature scales. The residence time

is the liquid mass divided by the process

input mass flow. Note that the jacket input

temperature loop is an inline loop where the

jacket temperature is a blend of a manipu-

lated makeup coolant flow with a constant

jacket recirculation flow where increases in

the manipulated jacket coolant makeup flow

result in corresponding changes in coolant

return flow by jacket outlet pressure control.

For jackets that require some heating, the

inline temperature controller manipulates

the steam to an inline injector and the return

flow is a hotter return water flow.

The secondary pH and temperature loops



are left to deal with the nonlinearities of the

process. They should be tuned for an ag-

gressive setpoint response minimizing rise

time even at the expense of an increase

in overshoot. Setpoint feedforward can

help where 50% of the inline loop setpoint

change is translated to the corresponding

change in inline PID output to achieve this

setpoint change. The setpoint feedforward

is simple added to the inline PID output via

the PID feedforward summer.

For both loops, the process gain is inversely

proportional to the inline flow which is kept

constant for vessel and jacket recircula-

tion flows. For these loops, a linear installed

flow characteristic is best. For inline pH and

temperature loops on vessel feed flows, an

equal percentage flow characteristic helps

compensate for the process gain being

inversely proportional to feed flow by a

valve gain being proportional to flow. Note

to insure the installed flow characteristic is

close to the inherent flow characteristic and

not distorted by changes in available pres-

sure drop, the ratio of valve drop to system

pressure drop should be greater than 0.25.

Providing enough pressure drop to the

control valve also prevents a severe loss in

valve rangeability, an aspect of the practical

reality due to deterioration from friction and

slope of the valve installed flow character-

istic near the closed position not discussed

by nearly anyone but me. Statements in the

literature and catalogs about valve range-

ability are generally flat out wrong.

The secondary pH and temperature loop

outputs go directly to the control valve. The

manipulation of a coolant or reagent flow

loop setpoint is not advisable because most

of the flow measurements used have insuf-

ficient rangeability. In general, the biggest

problem with cascade control systems that

manipulate a flow setpoint rather than a

valve positioner or variable frequency drive

speed, is the erratic flow measurement

signals at low flows. The signals get noisy

and some are simply set to drop out. You

can add logic to substitute an inferential

flow measurement based on valve position

using its installed flow characteristic but

this leaves the loop vulnerable to knowl-

edge of the installed flow characteristic and

precision of the valve. If you need to use a

flow loop, the best solutions are a magnetic

flowmeter or even better, a Coriolis mass

flowmeter, with a rangeability of 50:1 and

200:1, respectively. Differential head meters

with dual range differential pressure (d/p)

transmitters and vortex meters have a best

case rangeability of 15:1 that is difficult to

achieve in practice. A more typical range-

ability for dual d/p head and vortex meters

is about half the best case (e.g., 8:1).

The valve must be a true throttling valve,

not an on-off valve posing as a throttling

valve. A diaphragm actuator capable of pro-

viding 150% of thrust requirement should

be used with a goal of 0.2% resolution, 0.3%

deadband and 86% response time (T86) of

less than 4 seconds at a starting position of



10% besides 50%. A smart digital positioner

tuned with aggressive gain and rate action

is needed. The T86 performance and actua-

tor thrust objectives are relaxed here from 2

sec and 200%, respectively that were stated

in the March 2016 article ìHow to specify

valves and positioners that do not com-

promise controlî and the associated white

paper ìValve Response ñ Truth or Conse-

quencesî. Don’t be surprised if the valve

supplier will only provide the resolution,

deadband and T86 for a starting position of

50% because of the deterioration of these

metrics due to seating and seal friction near

the closed position.

The inline pH loop must still deal with the

titration curve. Signal characterization can

help to translate the controlled variable in

pH that is the Y axis to a % reagent demand

that is the X axis of the titration curve. The

linearization will not be perfect. A simple

standard signal linearization block where

you enter 20 or so X,Y pairs for a piecewise

linear fit is best. Since you are doing the op-

posite of the process for linearization, the X

value is the ordinate (pH) and the Y value is

the abscissa (reagent demand) of the titra-

tion curve. If just part of the nonlinearity is

addressed, you are way ahead in the game

in terms of tuning and performance. Plus,

the characterization reduces the noise seen

by the controller for setpoints on the steep-

er portion of the titration curve. Even with-

out such linearization, the inline pH loop can

usually correct for the disturbances within a

minute using mostly integral action.

For more on pH and temperature control,

see the ISA books ìAdvanced pH Measure-

ment and Control 3rdEdition” and ìAd-

vanced Temperature Measurement and

Control 2nd Edition”.

TO SUMMARIZE:1. Use cascade control of vessel pH and

vessel temperature to inline pH and inline

temperature control to provide a rela-

tively constant and easily estimated open

loop (process) gain that can readily pre-

vent the common occurrence of violating

the low PID gain limit for near integrating

and true integrating processes. Also more

aggressive tuning can be used by a more

linear and constant process gain seen by

vessel loop.

2. The inline pH and inline temperature con-

troller outputs should directly manipulate

precise and fast throttling valves with the

right installed flow characteristic.

3. The inline pH and inline temperature

controller should be tuned for an aggressive

setpoint response.

4. Signal characterization for the inline pH

loop PV using a piecewise linear fit should

be used to help deal with the nonlinearity of

the titration curve when the normal range

of inline pH setpoints are on the steeper

portion of the curve.



Living on the edge with surge controlBy Greg Mcmillan and Stan weiner, PE

Greg: I learned early in my career that compressor surge is the fastest phenomenon that

occurs in process industries around turbomachinery, and with the most disastrous conse-

quences. It’s like running up a hill and just as you get to the top, you come to a cliff, can’t

stop in time and go over the edge.

Stan: Naum Staroselsky was the first person to emphasize the precipitous drop in flow

during surge, the exceptional requirements as to how fast the automation control system

needed to respond, and that feedback control by itself often could not get a system out of

surge because the oscillations were so severe and fast.

Greg: I had the privilege of talking with Naum several times back in the 1980s. He developed

the essential speed-of-response requirements of each component in the system, as well as

algorithms for dealing with these fast transients. He also implemented strategies for com-

pressors operating in series to increase the total pressure rise and in parallel to increase the

total flow capacity. Here we have the opportunity to get a review and update with Naum’s

son, Serge Staroselsky, who is the chief technical officer for Compressor Controls Corpora-

tion (www.cccglobal.com).

Stan: Which types of compressors have the most severe surge cycles and damage?



Serge: Axial compressors are more suscep-

tible to damage due to their design. The

issue of protection from damage caused by

surge has always been very important to

anyone operating turbomachinery. Empha-

sizing this point, the latest edition of API

Standard 670, “Machinery Protection Sys-

tems” (5th Edition), has a dedicated section

on surge protection, which mandates a sep-

arate surge detector for axial compressors,

and recommends segregated detection for

critical centrifugal machines deemed sus-

ceptible to surge damage, such as compres-

sors operating at very high pressures (e.g.,

reinjection service).

Almost every compressor will eventually

suffer damage from excessive surge cy-

cles. In most cases, it’s continuous rather

than individual surge cycles that cause

mechanical damage. Our recommendation

is to always prevent three or more succes-

sive surge cycles in the time period on the

order of 10 seconds, whereas the guide-

lines by some manufacturers may be more

conservative, and use three or more surge

cycles in a time period as long as half an

hour. In many cases, even if there’s no

damage to the compressor due to surge,

there is a severe process upset that may

lead to lost production. The financial im-

plications increase if you’re talking about

more than one surge cycle, since the pro-

cess may not be able to recover, resulting

in equipment tripping and a subsequent

lengthy startup period.

Greg: What do you see as a loss in efficien-

cy from surge?

Serge: I have not seen much data on this.

We’ll be offering performance monitoring

software that includes the ability to calcu-

late the efficiency online, which will allow

us to evaluate the loss of performance over

time.

To do this, you need to know gas composi-

tion and use real gas equations; otherwise,

the monitoring system can’t provide suf-

ficient reliability. For many compressors

experiencing wear-and-tear due to surg-

ing for several years, the performance

curves (compressor pressure rise versus

suction flow curves for various speeds)

tend to move down and to the left, indicat-

ing internal recirculation. The compressor

produces a lower pressure at the same flow.

Of course, continuous and frequent surg-

ing can result in loss of performance within

weeks or even days; it just depends on how

often the machine surges and for how long.

Stan: How do you get an accurate surge

curve?

Serge: Our engineers start with the perfor-

mance curves from the original equipment

manufacturer (OEM) of the compressor.

It’s critical that we have a flow measure-

ment with good accuracy and repeatability.

A venturi tube can provide an excellent

low-loss measurement of compressor flow



given sufficient straight runs of pipe. Ori-

fice plates, V-cones, and Lo-loss tubes also

can provide sufficiently accurate and reli-

able flow measurement, provided they’re

installed following good engineering prac-

tices.

We highly recommend testing the compres-

sor in the field. The differences between

field and shop surge curves can be greater

than 10% due to inherent flow measure-

ment inaccuracies, as well as uncertainty

in the pressure and temperature measure-

ments at different operating conditions, and

limitations in the accuracy of the off-design

performance predictions of the OEM.

We like to do three surge tests at differ-

ent operating conditions to build the surge

limit curve. If there’s a major discrepancy

between field and OEM curves, we try to

find out why. Maybe the conversion of the

square root of the differential pressure (dP)

signal to flow is incorrect or there is a mea-

surement error. Our new diagnostic system

will provide an indication of whether the

operating point matches expected perfor-

mance and, in case of mismatch, indicate

probable causes, such as measurement

error. In any case, we report any discrepan-

cies to the user and OEM, and work with

them to identify the causes.

We try to make our recommendations

regarding proper selection and installa-

tion of instrumentation as early as possible.

In many retrofit jobs, we do a site walk

through the existing installation before even

starting the design of the surge control

system. It’s surprising how often good engi-

neering practices are not followed, creating

excessive delay, noise and erratic behavior

in the measurements.

Greg: How fast do the surge valves and

measurements need to be?

Serge: The surge valve 95% response time

(T95) for a full-scale signal to positioner

must be less than 2.5 sec. The time to go

from closed to completely open upon de-

energizing a solenoid valve must be less

than 1 sec. The hysteresis plus dead band

must be less than 1%. The maximum control

signal needed to initiate movement when

the valve is closed must be less than 3%.

Stick-slip must be negligible after initial

opening for a signal ramping at 0.3%/sec.

It’s important to do small-step as well as the

large-step tests.

The 63% response time (pure delay + time

constant) (T63) for the dP transmitters

should be less than 0.2 sec. and preferably

around 100 ms. Many leading manufacturers

have a version that will comply. Damping

settings must be removed or set to a mini-

mum. Any needed filtering is best done in

the controller. There are always pulsations

in the flow measurement. At least 0.1 sec

filter time constant is often needed. Some-

times noisier flows require a larger filter, but



we generally do not want to go higher than

0.3 sec.

Greg: What do you use as a trigger for an

open-loop backup when surge is imminent,

and how far open do you pop the surge

control valve?

Serge: The open-loop backup, which is a

step response to the valve position when

the operating point comes dangerously

close to the surge line, or its variations

have become the standard in the industry.

Without open-loop backup, you can’t have

a PID gain large enough to get sufficient

movement of the surge valve during a

large disturbance, yet maintain stabil-

ity during steady-state operation at low

loads when recycling is necessary. We

typically use a 10-15% margin between

the surge curve and the control line. The

step amplitude of the open-loop backup

is automatically adjusted based on the

strength of the disturbance. If the speed

of the approach to surge is relatively slow,

the surge valve may be stepped open by

10%. The step is made larger if the ap-

proach is faster. The main point is that we

try to avoid upsetting the process, and yet

still protect the machine. The small step

response to weak disturbances ensures

minimal impact on the process.

Greg: With any process control system,

you need to expect the unexpected, but

addressing any eventuality here is criti-

cal due to the speed of disturbance and

the potential damage and loss of produc-

tion. The valve response requirements are

exceptional. The use of volume boosters

without positioners in an attempt to make

a valve faster can be disastrous. Read-

ers are encouraged to thoroughly read

my treatise, “Valve Response—Truth or

Consequences” (www.controlglobal.com/

valve-response-truth-consequences).

Without open-loop backup, you can’t have a PID

gain large enough to get sufficient movement of

the surge valve during a large disturbance, yet

maintain stability during steady-state operation

at low loads when recycling is necessary.



The deal breakers26 things to not do in temperature, flow, valve, DP and pH applications

By Greg Mcmillan and Stan weiner, PE

Greg: Automation engineering is an especially challenging profession because you need to

get thousands of details right for a project to be successful, whether we’re talking about an

instrument upgrade and migration in an existing plant or a new greenfield plant.

Stan: Books and courses in academia address some of the control issues, but often on a

more theoretical level that’s oriented towards getting the next journal paper published

and the next research grant. Partnering with industry helps make the results more focused

and practical, but little is offered as to getting the right measurements, installation, control

valves, control strategy, filtering and all the PID details (e.g., PID form, structure, tuning and

execution rate).

Greg: You can read thousands of pages in books and handbooks written by practitioners,

but the info is buried at best. In many cases, what you need to know is not exactly any-

where. I’ve tried to convey practical information in my books, but have to admit the results

are overwhelming to the average engineer. Who has time to read and, even more impor-

tantly, categorize and organize the knowledge specifically needed? Hunter Vegas and

I have tried to address this in our ISA book 101 Tips for a Successful Automation Career

(www.isa.org/store/products/product-detail/?productId=116355), but much more needs to

be done.



Stan: Since we’re all pressed for time and

eliminating bad mistakes is the highest prior-

ity, we’re offering here, from our collective

100 years of experience, what can get you

into the most trouble. Dial thermometers,

pressure gauges and field switches shown in

some publications—like field engineers with

ties, no safety glasses and no hardhat—may

make a pretty picture, but don’t belong in

a plant. We don’t think we have to say such

stuff is dumb and even unsafe. If a tem-

perature or pressure is important enough

to measure or to trigger an alarm, it must

be done via smart transmitters so the signal

is reliable, seen by the operator and histor-

ized. Please insist that packaged equipment

suppliers adhere to this requirement and to

plant standards on selection and installa-

tion of instruments. Tell your vendors that

pushing cheap stuff and wearing suits and

ties make a bad instead of a good impres-

sion. There can also be “deal breakers,” even

when using smart instrumentation.

Greg: Here are my deal breakers for tem-

perature:

• A thermocouple instead of an RTD when

a repeatability better than 0.5 °C and a

drift less than 0.1 °C per year is needed

• Fit of sensor within thermowell has an air

gap around sheath or from sensor tip to

inside of thermowell wall or tip greater

than 0.02 in.

• A liquid or gas velocity less than 0.05

and 2 ft/sec past the sensor, respec-

tively

• An insertion length that doesn’t fully ex-

tend through the nozzle and is less than

6 in. past the wall or not within 25% of a

pipe centerline

• Using a DCS or PLC thermocouple or RTD

input card instead of a dedicated smart

transmitter with narrowed span and sen-

sor matching

Stan: I would say most orifice meters are

deal breakers for flow, but, acknowledging

there are some very large lines with mini-

If a temperature or pressure is important enough

to measure or to trigger an alarm, it must be done

via smart transmitters so the signal is reliable, seen

by the operator and historized.



mal performance requirements, here is my

list:

• Using an orifice meter when an accuracy

better than 2% of flow or rangeability

greater than 8:1 is needed (note that nor-

mal rangeability limit is 4:1, and can only

be stretched with a narrow-span trans-

mitter and an exceptionally uniform flow

profile and low noise).

• Using a vortex meter when accuracy bet-

ter than 0.5% of flow or rangeability greater

than 15:1 is needed (note that 15:1 is only

obtainable if the meter capacity matches

the maximum flow requirement, and there is

an exceptionally uniform flow profile)

• Using a magmeter when accuracy better

than 0.25% of flow or rangeability greater

than 50:1 is needed (note that 50:1 is only

attainable if meter capacity matches maxi-

mum flow requirement, and fluid conduc-

tivity poses no limitation)

• Using anything but a Coriolis meter when

accuracy of 0.1% of flow or rangeability

greater than 50:1 is needed

• Measuring with a meter body that’s not

completely filled with the fluid to be mea-

sured

Multiple inline meters of successively small-

er sizes can be used, but the additional

pressure drop, installed cost, maintenance

and complexity point toward using a single

flowmeter with the inherent rangeability

that meets the application requirement.

Greg: The most common cascade control

loop is a flow loop. Often ignored is the loss

of rangeability at low flows, causing noisy

and erratic flow signals. When the flow is

below the rangeability limit, the control

loop should use a flow computed from the

valve position, assuming the valve has mini-

mal backlash and stiction near the closed

position and good positioner sensitivity.

This brings me to my deal breakers for con-

trol valves. For more on bad deals for valves

see my May 2016 Control article, “How to

specify valves and positioners that don’t

compromise control” (www.controlglobal.

com/articles/2016/how-to-specify-valves-

and-positioners-that-dont-compromise-

control).

• Any valve originally designed for isolation

(tight shutoff)

• Any positioner with a poor sensitivity

that causes the 86% response time of a

valve to increase by more than 20% for

the smallest changes made in PID output

(spool-type positioners and many cheap

positioners are deal breakers)

• Any positioner tuned with integral action

• Any valve with a resolution or deadband



multiplied by the process gain PV, resulting

in a change in PV larger than allowable error

• Any valve with an 86% response time mul-

tiplied by maximum rate of change of the

signal resulting in a change in PV larger

than allowable error

Stan: Sensors and transmitters should be

chosen that minimize the length or elimi-

nate sensing lines without going to dia-

phragm seals. Given this is not always possi-

ble, here are some deal breakers:

• Differential pressure (DP) transmitter with

unknown fluid in sensing lines

• DP for flow or pressure measurement with

no equalization manifold

• DP for level measurement with no equal-

ization line on vessel that isn’t open to

atmosphere

• DP lines without continuous slope or or no

fill, vent and drain connections

• DP in gas service mounted below process

connections

Greg: There are so many deal breakers for

pH, I could write a book (wait, I did write

one). Here are the ones that first come to

mind:

• An electrode that sits dry for more than

20 minutes a day

• A fluid velocity less than 0.5 and 5 ft/sec

for clean and fouling service, respectively

• An electrode subjected to pH less than 1

or greater than 12 for more than 100 min-

utes a day

• An electrode in a pump suction line

• An electrode tip that isn’t pointed down or

isn’t within 25% of pipe centerline

Greg: I would like to say that not using

three electrodes with middle signal selec-

tion is a deal breaker, but this would be

too radical, even though in almost every

case, the increase in reliability and preci-

sion, reduction in noise and decrease in

maintenance make the return on invest-

ment ridiculously fast. Especially strange

is seeing owners of million-dollars-per-

day production units, which depend on

pH for product quality, go with just two

electrodes that raise more questions than

answers, but the owners don’t see their

way clear to using a third electrode.

There are so many deal breakers for pH, I could

write a book (wait, I did write one)



BIG BOOK TEMPERATURE

GUIDE

“The Engineer’s Guide to

Industrial Temperature

Measurement” is reported

to be the go-to guide for

reducing costs and improv-

ing performance. Its website

states, “The Engineer’s Guide

to Industrial Temperature

Measurement will ensure

you have the understand-

ing you need to select the

right measurement system.

It will also show you how to

install components correctly

and maintain the system.

The guide provides insight

beyond sensors, transmitters

and thermowells, offering

technical information on the

entire temperature measure-

ment system.” It includes

FAQs, temperature measure-

ment basics, engineering and

design, maintenance and

calibration, best practices

and reference materials. It’s

available free-of-charge, but

registration is required. It’s at

http://go.emersonprocess.

com/rmt-us-t-free-tempera-

ture-guide

Emerson Automation Solutions

www.emerson.com

REGULATORS 101 VIDEO

This 41-minute video, “Regu-

lators 101: Basics of Pressure

and Temperature Regulators,”

delivers a thorough examina-

tion of available technolo-

gies and their functions. It’s

presented by Jordan Valve’s

project manager, Harry Woe-

bkenberg, who discusses the

basics of industrial pressure

and temperature regulators.

Topics covered include: the

regulator–a proportional con-

troller; what to expect from

a regulator; keys to proper

regulator sizing; pressure-

reducing valves; back-

pressure-reducing valves;

and temperature regulators.

It’s at www.youtube.com/

watch?v=PrnCml6q4uM

Lesman Instrument Co.

www.lesman.com

TEMPERATURE CONTROL

PAGE

“Temperature Process Con-

trollers” webpage at Omega

Engineering’s well-known

website provides an introduc-

tion to many temperature

control concepts, tech-

nologies and products, and

includes links to many related

resources. It includes FAQs

and advice on evaluating and

selecting the right tempera-

ture controller, and is located

Resource guide: Temperature and pressure don’t stressA guide to web-hosted papers, programs, videos and other educational materials



at www.omega.com/prodin-

fo/temperaturecontrollers.

html

Omega Engineering

www.omega.com

DIFFERENTIAL PRESSURE

BASICS

This short, five-minute video,

“The Differential Pressure

Flow Measuring Principle

(Orifice-Nozzle-Venturi),”

by Endress+Hauser, pro-

vides a good introduction

to many of the pressure

principles involved in flow

measurement. It’s located

at www.youtube.com/

watch?v=oUd4WxjoHKY.

Endress+Hauser

www.us.endress.com

PRESSURE/TEMPERATURE

VIDEO

In just under three minutes,

this video, “Pressure and

Temperature Control-Basic

Applications,” touches on

the main concepts related

to pressure and tempera-

ture measurement in the

process control and auto-

mation field. It’s presented

by Danfoss Industrial Au-

tomation, and it’s located

at www.youtube.com/

watch?v=wIbeA9gMAzU.

Danfoss Industrial Automation

http://industrialautomation.danfoss.

com

CONTROLLER ESSENTIALS

“Temperature Controller

Basics Handbook” webpage

provides a short introduction

to the reasons why tempera-

ture controllers are needed,

how they work, common

applications, functional com-

ponents and types, and other

controller characteristics.

It’s presented courtesy of

Danaher’s Industrial Controls

Group and its Process Au-

tomation, Measurement and

Sensing division. It’s located

at www.instrumart.com/

pages/283/temperature-

controller-basics-handbook..

Instrumart

www.instrumart.com

COOL TEMPERATURE

BOOKLET

This 49-page PDF provides

a nice introduction to the

basic concepts of tempera-

ture measurement and most

of the various technologies

used to perform it. It also

contains a variety of useful

charts and references, and is

hosted and presumably pro-

duced by Missouri University

of Science and Technology.

It’s available at https://web.

mst.edu/~cottrell/ME240/

Resources/Temperature/

Temperature.pdf

Missouri S&T

www.mst.edu

PRESSURE, TEMPERATURE

LECTURES

These two one-hour videos,

“Lecture 4—Temperature

Measurement” and “Lec-

ture 5—Pressure, Force and

Torque Sensors,” present

many of the concepts es-

sential to both technical

fields. Subtitles are included

to fill in any gaps in under-

standing the audio. The

videos are part of an overall

series on Industrial Automa-

tion and Control by Prof.

S. Mukhopadhyay, Depart-

ment of Electrical Engineer-

ing, IIT Kharagpur. The first

is at www.youtube.com/

watch?v=As5kzxkyT24&i

ndex=4&list=PL874F91C0

180417C3 and the second

is at www.youtube.com/

watch?v=0MP_9n08urA

IIT Kharagpur

http://nptel.iitm.ac.in

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