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Feature Intelligent field devices

common process instrumentation application measures differential pressure in a steam line, with the final output being a flow rate measurement. The engineers or techni- cians who manage this application are concerned with accuracy and usually focused on the transmitter, which is a critical piece of equipment and impor- tant in providing an accurate output. However, the transmitter is only as accurate as the inputs provided to it. The process instrumentation loop – the set of tubing and components that con- nect the process to the transmitter – is just as important. The role of this loop is to present a set of process conditions to the transmitter. These conditions must be precisely the same as those in the process, otherwise the transmitter will not provide useful measurements. In the case of a differential pressure instrument loop, the layout and design of the system is especially complex. As with any process instrumentation application, you must pay careful atten- tion to the design and layout of the lines and the quality and installation of components. In addition, there are some critical maintenance issues. Any one of these matters can undermine the system’s capacity to provide accurate inputs to the transmitter.

Figure 1 represents a standard differ- ential pressure setup for measuring steam. The system consists of two pres- sure taps on either side of a known ori- fice that restricts flow in the process line. The pressure taps lead to a set of seal pots filled with condensate. This condensate serves as a liquid medium to send a pressure signal down the impulse lines to a transmitter. Based on the pressure difference between the taps and the known pressure drop of the orifice restriction, the transmitter yields a flow rate measurement. When it comes to the material choice of components, in a steam application, stainless steel or another corrosion- resistant alloy is strongly preferred over carbon steel. Still, many industrial plants employ carbon steel for process interface valves, for some piping, and even for manifolds (or parts of mani- folds). For example, it is not unusual to find carbon steel piping upstream of the seal pots – and also seal pots made from carbon steel – even if stainless steel tubing is employed downstream of the seal pots (Figure 2). Remember, the


Avoid weak points to ensure accuracy A

Eric Moore, technical manager, Swagelok Capital Projects Company, and Sam Johnson, process instrumentation products manager, Swagelok, explain the importance of the process instrumentation loop, in order to ensure an accurate reading in a differential pressure steam application

accuracy of your instrument loop can only be as good as the weakest point in the loop. Any wetted surface that is carbon steel can put the transmitter’s measurements at risk.

The first valve after the tap is the process interface valve (PIV), which enables the operator to isolate the process instrumentation loop for main- tenance. Many plants still employ a single gate valve or ball valve due to the sometimes high-temperature require- ments for the PIV and lack of suitably- rated alternatives. However, when temperature compatibility allows, the best choice for the PIV is a stainless steel double block and bleed (DBB), consisting of two isolation valves and one bleed valve, all in the same inte- grated unit. The reason for this is DBB valve promotes safety.

The seal pots serve the critical pur- pose of translating steam pressure into liquid pressure. Prefilled with conden- sate, the seal pots send a liquid pres- sure signal to the transmitter. Sometimes, engineers will piece together their own seal pots out of available materials and inexpensive valves. In many cases, these parts are carbon steel, which can lead to scaling and the contamination issues men- tioned above, especially when you con- sider the presence of vapor, air, and moisture – all of which promote corro-

Figure 1: A process instrumentation loop designed to measure differen- tial pressure

includes two tubing runs (taps), process interface valves, seal pots, impulse lines, blow down valves, an instrument

manifold, and a transmitter

sion – inside the seal pot. For this envi- ronment, prefabricated stainless steel seal pots with integral bleed valves are a better choice.

There are three main objectives when laying out the impulse lines, which connect the seal pot to the transmitter: • Prevent corrosion or scaling • Reduce leak points • Maintain temperature within a certain range

The first two are best achieved by employing tubing and tube fittings made from stainless steel or another corrosion-resistant alloy, as opposed to carbon steel pipe and threaded connec- tions. The third objective is achieved by heating the impulse lines.

The manifold is nearly as important as the transmitter itself for ensuring measurement accuracy. It is connected directly to the transmitter base and enables calibration and service of the transmitter. It is here, in the manifold, that scaling and corrosion from upstream components can lead to prob- lems that will undermine the accuracy of the transmitter.

Figure 2: It is a common, but ill- advised, practice to employ carbon steel pipes

upstream of the seal pots, even if stainless steel tub- ing is employed downstream of the seal pots

There are a number of reasons why one of the manifold’s needle valves could leak across the seat. Scaling or other system debris could lodge in the seat of the valve. The debris could damage the seat or prevent a seal between the seat and the needle tip. Burrs can produce the same effect. A third reason is poor design quality. The design of the metal-to-metal seat seal in the manifold’s needle valves is critical. Whether the stem tip is shaped like a ball or a vee, it is important that the tip not rotate with actuation. If it does, it may grind into the seat, scoring it and creating a pathway for leakage. Over time, the condensate in your system will need to be drained and replaced. In performing this routine service function, take care to avoid causing damage to the instrument loop and transmitter, which may result in lost efficiencies. You can do this by ensuring the draining process is moni- tored by technicians from start to finish. Also, when refilling the system with condensate, use a filling connec- tor, which mounts (with zero clearance) to the transmitter on the side that is opposite from the manifold. Also, ensure your filling connector employs a check valve.

By following these best practices, you can realise a low-maintenance, high-accuracy process instrumentation loop for differential steam pressure readings.


Enter 241 NOVEMBER 2013 Process & Control

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