5IM-P176-05 MI Issue 3

the measuring section.The orifice plate assembly should be fittedbetween two sections of straight cylindricalpipe of constant cross-sectional area.The pipe bore shall be circular over themeasured length.The inside surface of the measuring pipe shallbe clean and free from scale, pitting and depositsor any other possible source of turbulence.The pipe and pipe flanges should be laggedfor steam applications.The recommended minimum upstream anddownstream lengths for optimum accuracyare shown in Fig 3. Shorter lengths can beused although these will reduce the accuracyof the system.When the pipework is ready for the installationof the orifice plate assembly, remove theassembly from the packing and check theserial number against that supplied on thedrawing. As the orifice plate is machined tohigh tolerances care should be taken whenhandling to prevent damage.The orifice plate carrier assembly is heldtogether by tape and this should be checkedto ensure that it is secure.

3.2 Installation of the M410orifice plate assembly

The orifice plate assembly consists of an orificeplate, optional carrier rings with inch NPTtapping and gaskets to fit between the orificeplate and carrier ring, and the carrier ring and theflanges. This assembly is held together by tape.The orifice plate assembly should be installedbetween the pipe mating flanges as shown inthe diagram below ( Fig 4 ). It is important toensure that the 45 chamfer should bedownstream, this will be achieved when theengraving on the tab handle of the orifice plateis facing in the upstream direction.When installed in this way the high pressuretapping in the carrier will be upstream of theorifice plate. Care should be taken to ensurethat these are connected to the correct port

on the 3-valve manifold on the DP transmitter.The outside diameter of the orifice plate andcarrier assembly is equal to the flange boltPCD minus the diameter of the bolts to ensureprecise centring of the orifice plate in thepipeline, although care should be taken toensure that the gaskets do not protrude intothe measured pipe length. Where a drain holeis present in the orifice plate, ensure that thisis positioned at the lowest point to avoidcondensate or liquid build-up behind the plate.

3.3 Installation in steam pipelinesThe following instructions apply for the layoutof the orifice plate and transmitter when steamflow is being measured.The position of the pressure tappings shall behorizontal or up to 45 above the horizontal.

Fig 6 Recommended layout of system

See figure 5The DP Transmitter should be installed at alower position than the orifice plate. The impulselines should be filled with condensed water toprevent damage to the transmitter. Temperatureat the measuring cell should be less than100C.The impulse piping should have a minimumfall of 1:20, run over the same route preferablyclipped together and unlagged. It should havea minimum O/D of (12.7mm) and be of asuitable pressure and temperature rating forthe application.The impulse piping should follow a route to preventany elbows where air or non condensable gasescould accumulate. This could cause seriouserrors in the DP Transmitter reading.The recommended layout of the system canbe seen in Fig 6

Fig 4

Flow

Pipe flanges

Gaskets

Carrierrings

Squareedge upstream

Fig 5 Position of Pressure Tappings forSteam Applications

4545

Isolation valves

3-valve manifold

Impulsepiping

Min fall of 1:20

Min inside diameter " (12.7mm)

Transmitter

Orifice plate engravingin upstreamdirection

!"#$%$&'$$

!

!

"##

$%

The Steam and Condensate Loop4.4.2

Instrumentation Module 4.4Block 4 Flowmetering

Instrumentation

A steam flowmeter comprises two parts:

1. The primary device or pipeline unit, such as an orifice plate, located in the steam flow.

2. The secondary device, such as a differential pressure cell, that translates any signals into ausable form.

In addition, some form of electronic processor will exist which can receive, process and displaythe information. This processor may also receive additional signals for pressure and/or temperatureto enable density compensation calculations to be made.

Figure 4.4.1 shows a typical system.

Fig. 4.4.1 A typical orifice plate steam flowmetering station

Fig. 4.4.2 Simple DP cell

Differential pressure cells (DP cells)

If the pipeline unit is a differential pressure measuring device, for example an orifice plate flowmeteror Pitot tube, and an electronic signal is required, the secondary device will be a DifferentialPressure (DP or DP) cell. This will change the pressure signal to an electrical signal. This signal canthen be relayed on to an electronic processor capable of accepting, storing and processing thesesignals, as the user requires.

Temperaturetransducer

DP cell andtransmitter

(secondary element)

Flow

Upstreampressuretapping

Downstreampressuretapping

Pressuretransducer

Flow processoror computer

Orifice plate assembly(primary element)

Upstreampressure cap

+ DP cell -

Measuringdiaphragm

Dielectric oil filling

Measuring cell

Isolatingdiaphragm

Output

Downstreampressure cap