Feature Test & measurement


A standard solution for accurate measurements


Measurement accuracy is as dependent on the accuracy of the transmitter as it is on the quality of the instrument loop components. Here, Eric Moore, technical manager at Swagelok Capital Projects Company, and Sam Johnson, process instrumentation products manager, Swagelok, explain how standardization will raise the accuracy of your measurements


often be accuracy. The transmitter is a critical piece of equipment here, so the engineer will usually purchase the most accurate one they can, and dedicate a great deal of attention to it. The trans- mitter, however, is only as accurate as the inputs provided to it.


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The process instrumentation loop – the set of tubing and components that connect the process to the transmitter – is just as important. This loop presents a set of process conditions to the trans- mitter. The conditions, however, must be precisely the same as those in the process; if not, the transmitter will not provide useful measurements. However, it can be difficult to know when the process instrumentation loop is not performing well. So, while the engineer or technician may be focused on the transmitter, the instrument loop may be the cause, undermining any possibility of success.


It is therefore essential to educate 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 To start with, let us look at an alterna- tive that has recently been developed. The usual process instrumentation setup entails – at a minimum – a process interface valve, impulse lines, a manifold, and a transmitter (Figure 1). Impulse lines can be costly to install and maintain, and face challenges such as clogs, leak points, temperature control, and corrosion.


Close coupling eliminates the impulse lines. The process inter- face valve and manifold become one unit (Figure 2), and the transmitter mounts directly to it. The entire assembly then attaches to the process line. The challenge here is


Instrumentation NOVEMBER 2013


hen measuring pressure, flow, or level in process instrumentation applications, the engineer’s top priority will


and location of the valve).


Standardization can also mean simplicity – including maintenance, installation, training, diagnostics and a reduction in error. The facility can stock fewer replacement parts.


Valves


Figure 1: The process instrumentation loop consists


mainly of a process interface valve (PIV), impulse line and fittings, a manifold and the transmitter


finding the right places to use it. A limitation is temperature. One reason for the traditional setup with impulse lines is to protect the transmit- ter 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.


Furthermore, 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.


Figure 2:


A close couple eliminates the impulse lines. The process interface valve and manifold become one unit, and the transmitter mounts directly to it


Close coupling also 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.


In the loop


If the goal is an optimal design, there are a limited number of ways to set up the process instrumentation loop. All systems, ideally, should be designed using a consistent set of criteria, includ- ing budgets and allowances for down- time, maintenance and accuracy, with the result being a high degree of standardization.


Before standardization, a refining plant may have had 30 different config- urations for process instrumenta- tion loops, but after it 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,


For each of the basic building blocks in a process instrumentation loop – the process interface valve, the impulse lines and the manifold – there are criti- cal 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. Many industrial plants still employ carbon steel for process interface valves, for some piping, and even for some manifolds (or parts of manifolds). However, the scaling that commonly builds up on carbon steel can break away, flow downstream, lodge in a valve seat, and prevent a positive shutoff. The result is an inaccurate transmitter calibration and/or inaccu- rate transmitter readings. If carbon steel components are used in the instrument loop, they will require very close monitoring.


The process interface valve (PIV) is the first in the process line. Historically, the PIV of choice has been a single gate valve or ball valve. While both are still in use today, 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 buildup in the process instrumentation loop. Although the function of a DBB can be constructed by using three separate valves, the better option is a single, self-contained unit, which has a number advantages: • Fewer leak points • Flexibility to configure the product using different size orifices and flanged connections;


• Reduced size and weight, reducing the need for – and expense of – struc- tural support of the instrumentation system (Figure 3)


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