Dec-2013

Ensuring an accurate reading in a differential pressure steam application

A common process instrumentation application measures differential pressure in a steam line, with the final output being a flow rate measurement. The process instrument engineers or technicians who manage this application are concerned with accuracy and, typically, are focused on the transmitter, 
which is a critical piece of equipment and 
very important 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 connect 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. If they’re not, 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 attention 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.

Eric Moore and Sam Johnson
Swagelok

Article Summary

Designing process instrumentation loops
Figure 1 represents a standard differential pressure setup for measuring steam. The system consists of two pressure taps on either side of a known orifice 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.

The set of components entailed in delivering condensate to the transmitter consists, at minimum, of two process interface valves, the impulse lines (and connections), two blow down valves, and a manifold.

The design and layout of these components should be as consistent as possible throughout your plant. Standardisation reduces the opportunity for error and makes many activities simpler, including maintenance, installation, training, and diagnostics. In addition, with standardisation, fewer replacement parts need to be stocked.

The choice of materials for the components in your system is very important. In a steam application, stainless steel or another corrosion-resistant alloy is strongly preferred over carbon steel. This rule applies to all wetted parts of the components because corrosion (“scaling”) in carbon steel components and pipe can disrupt the proper functioning of valves downstream, which can result in inaccurate inputs to the transmitter. We’ll discuss this in more detail shortly.

Still, many industrial plants employ carbon steel for process interface valves, for some piping, and even for manifolds (or parts of manifolds). 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 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, regardless of its location in the system. If you employ carbon steel components, they will require very close monitoring to ensure that scaling is not affecting the operation of valves downstream. Keep in mind, too, that there is no easy way to know if a valve is leaking. There is no alarm that sounds, so you may only know because your measurements are not consistent.

The first valve after the tap is the process interface valve (PIV), which enables the operator to isolate the process instrumentation loop for maintenance. Many plants still employ a single gate valve or ball valve due to the sometimes high-temperature requirements 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 integrated unit. A DBB valve promotes safety. If the first block valve were to leak, the second block valve would prevent pressure or fluid buildup in the process instrumentation loop. Although the function of a DBB valve can be constructed by using three separate valves, single, self-contained units are preferred, as they reduce size, weight, and potential leak points (Figure 3).

The seal pots serve the critical purpose of translating steam pressure into liquid pressure. Prefilled with condensate, the seal pots send a liquid pressure 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 mentioned above, especially when you consider the presence of vapour, air, and moisture — all of which promote corrosion — inside the seal pot. For this corrosion-heavy environment, prefabricated stainless steel seal pots with integral bleed valves are a better choice.

Three main objectives come into play 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 connections. Stainless steel tubing can be bent and shaped, which reduces the number of mechanical connections. When mechanical connections are necessary, high-quality two-ferrule, mechanical-grip type tube fittings are preferred, as they will not back off with thermal cycling or vibration, unlike threaded fittings used with carbon steel.

The third objective — maintaining temperature within a certain range — is achieved by heating the impulse lines. You can insulate your impulse lines manually — “field tracing” — or purchase tubing that has been insulated in the factory and encased in a polymeric jacket.

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 problems that will undermine the accuracy of the transmitter.

The manifold in a differential pressure application consists of at least three valves: two needle valves to isolate the transmitter from the rest of the instrument loop and a third needle valve for equalisation of pressure between the two differential pressure lines during calibration. During calibration, the two isolation valves in the manifold are in the shutoff position. If the shutoff is less than complete, the result is an inaccurate reading from the transmitter.