Reciprocating Meter Pumps in Leak-Free

wl Des ign

Diaphragm designs are proving their worth in critical fluid handling processes as well as everyday applications. By Gerald Harting

t is hard to pick up a technical journal nowadays without see- ing evidence of the increasing trend towards zero-leakage pumps. Although driven in part by environmental and safety requirements, the justification is I often primarily economic - that

is, plant managers want to improve reliability and process integrity while minimizing downtime, scrap losses and maintenance. Leak-tight pumps often have a much higher ini- tial cost than equivalent sealed designs, but the payback can be short if they are properly applied.

The most common rotating pump is the centrifugal, with the sealless designs incorporating canned motors or magnetic drives to deliver torque to the impeller without the need for moving seals. In reciprocating pumps a mechanically or hydraulically actuated diaphragm is used to replace sliding plunger seals. Although these pumps are called "sealless," they do have static seals at component interfaces, such as the can in the centrifugal or the diaphragm clamping area in the diaphragm pump. Reciprocating diaphragm pumps in single and multi- head configurations are available for metering and controlled volume ser- vices over the remarkable power range of 0.1-1000 hp and they are well proven. They offer special advantages in the pumping of liquids that are cor- rosive, flammable, reactive, non-lubri- cating, abrasive, toxic, shear-sensitive, high purity, and sterile (medical or pharmaceutical). And they may be appropriate as well for fluids that are costly to produce. The application areas are very broad and include the indus- tries producing chemicals, oil & gas, personal care products, detergents,

24 APRIL1996

paper, plastics, food and beverages, pharmaceutical, and power, as well as the subset of water and waste treat- ment applications within each of these industries. Due to their exceptionally long up-times, high efficiency, zero emissions, and their simple, quick maintenance, the trend toward use of diaphragm pumps is extending even into the handling of liquids formerly considered not critical. Water and waste treatment applications for meter- ing pumps represent 65% of the mar- ket, and are well known and straightforward services. Therefore, the focus of this article will be on the other 35% of the market for metering pumps, industrial and process applications. The article will explain how modern diaphragm designs function, and it will detail their performance characteris-

tics, installation requirements, and safety advantages.

RECIPROCATING DRIVE ELEMENTS (FIGURE 1)

Diaphragm pumps employ the same reciprocating mechanisms as plunger pumps. Simple applications utilize a simplex drive element, usually with adjustable stroke length to vary the flow from the pump by changing the displaced volume (Figure la). Such drive mechanisms must be stroke adjusted manually, electrically, or pneumatically under full and pulsating load, so even modern designs with excellent internal load management are generally limited to about 50 hp. Drive elements without stroke adjustment are capable of much higher hp inputs. Modular drive element designs can be

monoblock diaphragm pump delivers 8,750 gph at 2,200 psi with 150 hp input.

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four identical barrel pumps, one unit persistently vibrated. The motor and even the pump were replaced with still no improvement. The investiga- tion, however, identified that the grout under the barrel flange was totally inadequate for the vibratiTg pump. In other words, weaker su - port put the pump in a natural f rq , quency mode equal to the running\ speed. This has been seen in other installations where pumps were mounted on beams or slabs that hap- pen to lower natural frequencies to the pump speed.

When using variable speeds, the pump may be in a natural frequency mode at one of the speeds. If so, it is best to block out that area of speed in the controls.

Operation. When pumps are selected, built and installed properly, they will have a reasonable life expectancy. If a pump's life is not up to reasonable expectations, its selec- tion, use, construction and installa- tion must be thoroughly checked. Performance can be changed to suit head capacity requirements; con- struction and materials can be upgraded as necessary; and bearing systems, including the thrust bearing systems in the driver, can be improved. Many vertical pumps will start in upthrust. So, the bearing sys- tem must be built to handle that, and in some cases the pump may be required to run in continuous upthrust. The system can be built for that, too.

Sometimes particular pumps continually vibrate or rattle or run

quently. That's when time must out and repairs have to be

taken to solve the problem whatever the problem is - to the pump, head, materials and between!! W

Herman Greutink is Vice Presi- dent and Technical Director for John- ston Pump Company (Brookshire, TX). Among other professional affiliations, he is a longstanding member of Texas A&M's International Pump User's Symposium Advisory Committee and the Hydraulic Institute, and he fre- quently contributes to Pumps and Sys- tems on the subject of vertical turbine pumps.

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multiplexed together to achieve process size flow ranges, with each drive ele- ment equally phased from the other to deliver smoother discharge flows. The trend is to horizontally multiplexed designs [as opposed to vertical) for mechanical, lubrication, NPSHR and space efficiency reasons. As process flow rates become larger, a compact monoblock triple-crank drive becomes more economical, does not require a baseplate, and is mechanically more reliable. Flow is adjusted over the required range by a variable frequency drive. Power ranges from 10 to more than 750 hp are common (Photo 1).

MECHANICALLY ACTUATED DIAPHRAGM DESIGNS

These pumps are well proven and reliable. If care is taken in their selec- tion and application, they will perform extremely well to specifications. The diaphragm forms a flexible barrier, totally containing the process fluid and separating it from the reciprocating dri- ve element, which is attached to the diaphragm at the center with a clamp- ing disk. A hermetic seal is made at the circumference of the diaphragm and at the clamping disk. Because the diaphragm is simultaneously loaded by deflection as well as by the pressure generated by the pump, these designs are limited to pressures below 300 psi. The force acting on the plunger rod is proportional to the fluid pressure and approximately to the area of the pusher disk in contact with the diaphragm, so relatively large drive thrusts are required. Conservative designs limit powers to 2 hp or less. Cam-and-spring (lost motion) drive mechanisms are often used in the smaller sizes (Photo Z), with variable eccentric (amplitude modulated) drives on larger sizes. In designs that use elastomeric dia- phragms, the process pressure can deform the diaphragm and reduce the metering performance of the pump. Recent developments to overcome this problem include a multi-layer pure PTFE design [Photo 3) that offers broad chemical resistance, condition monitor- ing, a built-in backup diaphragm, and a stiffness against process pressure and service lifetime approaching that of a hydraulically actuated design. The major advantages of mechanically actu- ated diaphragm pumps are their sim- plicity (no hydraulic system required), high suction lift capability, and low first cost. Modern designs should be consid-

ered for low pressure processes where their advantages can be decisive.

HYDRAULIC DIAPHRAGM DESIGN BASICS

The diaphragm totally contains the process fluid, and the separate reciprocating plunger operates in hydraulic fluid. The volume of con- fined hydraulic fluid displaced by the

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moving plunger causes flexure of the diaphragm and the equivalent dis- placement of process fluid. There- fore, the freely moving diaphragm is stressed by deflection only, and there is equilibrium of pressures on both sides of the diaphragm and pressure uniformity across the full face of the diaphragm. The only moving parts in the process stream are one-direction

~ ~~~

PUMPS AND SYSTEMS MAGAZINE 25 Circle Number 218

With Stroke Adjustment i

a

Without Stroke Adjustment

b

Figure 1. Drive element configurations: a, by cy and d (side view) can be multiplexed up to twelve. Monoblock triplex e (top view).

automatic check valves to create an upward flow through the pump head, a fluid motion that follows plunger oscillation. For such a design to work reliably for extended peri- ods, a number of conditions must be met:

1. Constant volume of hydraulic fluid in the pressurized circuit by automatic leakage replenishment.

2. Continuous venting (degassing) of any vapor in the hydraulic fluid.

3. Relief of excess pressure to avoid overloading.

4. Protection of the diaphragm against cavitation, or excessive stresses from high suction pressure

26 APRIL1996

or improper operation of discharge valve.

Additional important features of this kind of pump preferred by users include:

1. Multi-layer diaphragm for sec- ondary containment.

2. Continuous monitoring of diaphragm condition.

3. Continued operation on one diaphragm layer for controlled shut- down or switch over to a standby pump without process interruption.

4. Fast and simple maintenance for minimum downtime.

Modern diaphragm pumps pro- vide all of these features using either

Photo 2. Cross-section of a heavy duty mechanically actuated diaphragm meter- ing pump using a cam-and-spring drive element.

I 16

Photo 3. Multi-layer PTFE diaphragm with two working layers, monitoring layer, and backup diaphragm (U.S. patent 5,074,757).

PTFE diaphragms for pressures up to 5,000 psi or metal diaphragms for pressures to 17,000 psi and higher. While there are many diaphragm shapes and designs, the most com- monly used are plain flat diaphragms which are considered optimal with respect to quality, economy and ease of use as well as the design of the diaphragm support.

HOW A PUMP HEAD WITH PTFE DIAPHRAGM WORKS Pumping Stroke (Figure 2)

The process fluid (3) is totally contained by the rigid diaphragm cover (1) and the flexing pumping

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chamber wall formed by the hydraulically balanced PTFE dia- phragm (2) , which is clamped with a hermetic seal at its circumference. This prevents any moving or sliding seals from being wetted by the process fluid. The displacer is a pack- less plunger (4), which operates in clean hydraulic fluid with good lubri- cating properties and relies on piston rings to eliminate seal maintenance and adjustment. The volume of con- fined hydraulic fluid (5) displaced by the plunger on the forward stroke causes forward flexure of the diaphragm and the equivalent dis- placement of process fluid. One directional check valves (6, 7) create an upward flow through the pump head. On every forward stroke of the plunger, the vent valve (9) volumetri- cally "bleeds" a metered amount of pressurized hydraulic fluid back to the reservoir to remove any vapor bubbles that may form due to pres- sure and temperature fluctuations. This assures a stiff hydraulic system. At the same time a small, consistent slippage at the piston rings lubricates and cools this critical area for long service life. The built-in pressure relief valve (8) protects the pump against overload due to dead-head- ing. Also, since it functions in clean hydraulic fluid, it provides a reliable backup to a process side PRV, pro- tecting the system from pump-gener- ated over pressure. Suction Stroke (Figure 3)

As the plunger moves rearward, the hydraulically coupled diaphragm follows rearward, dropping the pres- sure in the pumping chamber (3) to suction pressure and refilling this con- trolled volume with process fluid. On the hydraulic side of the diaphragm, the hydraulic fluid volume -being slightly reduced due to the metered venting and controlled slip at the piston rings - allows the diaphragm to reach and rest against the smooth, solid back- up surface of the pump body (Photo 4) before the plunger reaches its rearmost position. In this position the diaphragm is fully supported, and even pressures as great as 5,000 psi (resulting, for example, from high suction pressure or failure of the discharge valve) will not cause perforation or damage. The diaphragm mechanically opens the centrally located gate valve ( l o ) , per- mitting the refill valve (1 1) to replenish the stroke volume of hydraulic fluid

precisely as the plunger continues its travel back to its rearmost point.

DIAPHRAGM POSITION CONTROL Diaphragm Position Control (DPC)

allows a freely oscillating diaphragm to be safely used, eliminating the need for a front perforated support plate. The arrangement of gate and refill valves shown controls the position of the

diaphragm by continuously maintaining the correct hydraulic volume without overfilling. DPC always forces the diaphragm to start from the full backup support position, thus keeping it safely away from the inside of the diaphragm cover plate. In this way the diaphragm is also protected from any damage that might result from particles in the process stream. During periods of par-

PUMPS AND SYSTEMS MAGAZINE Circle Number 219

27

ture and is immediately sensed with simple, reliable devices that can trip alarms locally, remotely or both. Failure of both diaphragms simultaneously is rare and usually occurs only when there is an extreme mechanical difficulty, such as solidification of the process fluid. "Sticking" the two diaphragms together by hydrostatic adhesion is a proven method of assuring that the diaphragms

act as one even under the most difficult suction conditions. In this design, often called a "sandwich," any clear fluid com- patible with the process is introduced between the two diaphragms at the time of pump head assembly, and all but a very thin film is squeezed out through a non-return valve upon the first few strokes of the pump. (The volume of flu- id film in Figures 6 and 7 is overstated

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for clarity.) The film's important duties include holding the diaphragms together to act as one, enabling them to flex with- out abrading each other, and the rupture detector is prefilled for response with a few strokes. Vacuum coupling of two diaphragms is also widely used, but the normal drop in vacuum during opera- tion can reduce the effectiveness of the coupling and pump efficiency.

Once diaphragm perforation is sensed and indicated, nothing else hap- pens-the pump continues to run at full performance and leakage free. Mainte- nance can be scheduled when conve- nient, switch-over procedures can be manually or automatically implemented to an on-line spare pump, and the pump to be shut down can be flushed with a neutral fluid, if appropriate. Even if both diaphragms of the sandwich are run to failure, the process is still contained. Process fluid can enter the hydraulic system but is prevented from leaking at a rate any more than a seepage from the breather (12) as it is stopped by the vent valve (9), packless plunger (4) , and gate valve (10). In this case, a pressure rise detector and shutoff valve at the breather is a reliable "fail-safe" solution.

DIAPHRAGM ENDURANCE The operating life of a diaphragm is

influenced by many factors, the most important being correct selection of the pump and design for the service, proper installation in a suitable piping system, and operation in accordance with manu- facturer's recommendations. Studies of up-times for a large range of pumps show that PTFE dia-phragms last to 20,000 hours or more in continuous, high pres- sure operation at pump speeds of 200 strokes per minute. Metal diaphragm designs can operate 8,000-10,000 hrs or more continuously, even at their highest operating pressures. Naturally, intermit- tent operation will lengthen diaphragm lifetimes to many years, provided proper startup procedures are followed. Shorter

mechanical damage by the process fluid (solidification), induced mis-operation of the hydraulic system (e.g., wrong or dirty hydraulic fluid) , or piping system prob- lems such as excessive stresses from cav- itation, pulsation or overload.

PUMP PERFORMANCE CONSIDERATIONS

The volumetric efficiency of diaphragm pumps at high pressures is lower than for plunger pumps due

~

lifetimes can normally be attributed to -

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to the compressibility and elasticity effects in the necessarily larger dead volumes in the discharge and hydraulic fluid working spaces. Ener- gy efficiency, however, is typically better for diaphragm pumps as the high friction of plunger sealing is eliminated. This also eliminates the added heat in the process fluid, and it prevents particles of the seal from contaminating the process stream.

Another energy advantage of diaphragm pumps is that a "start-up device" can be installed to recirculate the high pressure hydraulic fluid during start-up until the drive motor comes up to speed. This allows the pump to start without experiencing the process pres- sure and without use of the oversized motor and VIS frequency inverter that would otherwise be required. By-passing on the hydraulic side is also more reliable than on the process side, especially with slurries. The lower volumetric efficiency of diaphragm pumps at high pressures (i.e., 70-90% for diaphragm pumps vs. 90-96% of plunger pumps) requires that more attention be given to the piping sys-

tem and dampening devices, as pulsation and vibration excitation, even with mul- tiplex pumps, can be significantly higher. The technologies of piping analysis and system interface devices (directional dampeners for flow, pressure and noise attenuation) has advanced to the point where degrees of smoothness approach- ing that of centrifugal pumps can be reli- ably predicted and achieved.

Suction conditions with dia- phragm pumps also must be thor- oughly considered. NPSHR for diaphragm pumps is not much dif- ferent from that of plunger pumps, but internal hydraulic losses must be added. Pump designs with perforat- ed support plates will naturally have higher losses. For continuous safe operation, diaphragm pumps with DPC should not be subjected to suc- tion conditions below 8.5 psia.

CONCLUSION Diaphragm pumps for metering

and process applications have devel- oped to a high degree of reliability, safe- ty and economy and they are available

in a wide performance range. The suc- cessful application of diaphragm pumps requires a knowledge of their function, limitations and characteris- tics. Partnering with a supplier that can provide a full range of products, refer- ences and services such as piping sys- tem analysis and recommendations (including system interface devices such as dampeners) as well as in-depth technical support is an important first step to achievement of design goals and overall lasting satisfaction.

. ~_______

Gerald Harting is Vice President and General Manager of AMERICAN LEWA, INC., Holliston, MA. He holds degrees in Mechanical Engineering from Wentworth Institute of Technolo- gy and Northeastern University, and has held positions in design, product development, sales, and marketing management. He has 33 years' experi- ence with metering and process pumps and packaged systems using reciprocat- ing metering pumps and rotary gear pumps.

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