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Jan-2014

What your transmitter may not be 
telling you

If you value accurate process measurements and specify a premium transmitter, your instrument loop requires the same level of attention

ERIC MOORE, Swagelok Capital Projects Company
SAM JOHNSON, Swagelok

Viewed : 3353


Article Summary

In process instrumentation applications measuring pressure, flow or level, the engineer or technician is focused on accuracy as a top priority. And a critical piece of equipment is the transmitter. Engineers will usually buy the most accurate one they can and dedicate a great deal of attention to it.

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 are not, the transmitter will not provide useful measurements.

Further, it is often hard to know when the process instrumentation loop is not performing well. There is no alarm that sounds. So while 
the engineer or technician 
may be focused on the transmitter, the instrument loop may be the cause, undermining any possibility of success.

Therefore, there is good reason to be educated about the possible issues in a process instrumentation line, including those related to overall design and layout, as well as individual component quality and installation.

Close coupling
Before we review the process instrumentation loop in detail, let us look first at a recently developed alternative. It is an elegantly simple solution, and if your application allows you to employ it you should.

The usual process instrumentation setup entails – at a minimum – a process interface valve, impulse lines, a manifold and a transmitter (see Figure 1). Impulse lines can be costly to install and maintain, with challenges like clogs, leak points, temperature control and corrosion. Close coupling eliminates the impulse lines. The process interface valve and manifold become one unit (see Figure 2), and the transmitter mounts directly onto it. Then, the entire assembly attaches to the process line. Almost everyone who learns about this alternative likes it. The challenge is finding the right places to use it.

One limitation is temperature. One reason for the traditional setup with impulse lines is to protect the transmitter from the high temperature of the process line. If the process is too hot, the transmitter may not be able to operate only a few inches away in a close coupled installation.

A second limitation is access. If you need to get to the transmitter for calibration, it needs to be accessible, so mounting a close couple on a process location 50ft in the air does not make a lot of sense.

The only other obstacle is cost. Close coupling requires an initial investment, but in the long run it may be less costly, especially if you figure in the low cost of maintaining a close coupled system, as compared to the traditional alternative. If you have an opportunity to employ this solution, you should do it.

Standardisation: a step towards simplified maintenance
If your goal is an optimal design, there are a limited number of ways to set up the process instrumentation loop. And yet, in most plants, there is a multitude of variations, and many of them are not optimal. These variations have been developed over time, by different engineers and/or contractors for different projects and reasons. Such a situation can be a drain on your time and energy. Each system has different needs in terms of maintenance, and when things go wrong there are a multitude of possibilities.

Ideally, all systems should be designed using a consistent set of criteria, including budgets and allowances for downtime, maintenance and accuracy. The result is a high degree of standardisation. For example, before standardisation, a refining plant may have 30 different configurations for process instrumentation loops. After standardisation, the same plant may have only five, with each containing the same basic components: a transmitter mount, manifold system and redundant pressure measurement. The only variations might be the tubing runs and the type of process interface valve (based on temperature, pressure and location of the valve).

With standardisation, many things become simpler, including maintenance, installation, training and diagnostics. Error is also reduced. In addition, the facility can stock fewer replacement parts, reducing overheads.

Basic building blocks
For each of the basic building blocks in a process instrumentation loop – the process interface valve, the impulse lines and the manifold – there are critical choices in terms of materials and design that can affect accuracy.

Regarding materials, stainless steel or another corrosion resistant alloy is strongly preferred in most applications. Still, many industrial plants employ carbon steel for process interface valves, for some piping and even for some manifolds (or parts of manifolds). In certain low moisture applications, like oil, carbon steel is acceptable, but for most other applications it can be a risk. The scaling that commonly builds up on carbon steel can break away, flow downstream, lodge in a valve seat and prevent a positive shut-off. The result is an inaccurate transmitter calibration and/or inaccurate transmitter readings (as we discuss in more detail below). If you employ carbon steel components in the instrument loop, they will require very close monitoring to ensure that scaling is not affecting the operation of the valves in the system.

Process interface valve
The process interface valve (PIV) is the first valve off the process line. Historically, the PIV of choice has been a single gate valve or ball valve. Both continue to be used today, especially in the US, but the best practice is a double block and bleed (DBB) valve, which consists of two isolation valves and one bleed valve in between. The main reason for employing a DBB is safety. If you need to shut down the process instrumentation line for maintenance, you would close both block valves and open the bleed valve. If for any reason the first block valve were to leak, the second block valve would prevent pressure or fluid build-up in the process instrumentation loop.


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