There’s a wide range of accessories for your pressure instruments that can help make your process safer, more efficient and precise. Snubbers, dampeners, capillaries, pressure limiting valves, siphons and other accessories are common additions to the most challenging applications.
One question I’ve heard often during my time in this industry is: do these accessories impact the accuracy of pressure measuring instruments?
In this article, I will discuss many of the common pressure instrument accessories available today, how they work, and how their use may or may not impact the accuracy of your measurements.
What Factors Impact Accuracy of Pressure Instruments?
First, let’s dive into the issues that can influence accuracy. There are many ways that accuracy can be impacted on pressure instruments, which we have already detailed in our “Instrument Killers” article.
These factors include:
- Excessive pulsation/vibration
- Extreme low/high temperatures
- Pressure spikes
- Instrument clogging
- Instrument corrosion
Next, we’ll go deeper into these factors and discuss the accessories and instrument assemblies designed to help counteract these negative effects on your pressure instrumentation. Knowing how they work can help you understand how they may impact accuracy.
What Are the Different Pressure Instrument Accessories?
The ASME B 40.100 Standards Guide identifies several types of accessories that protect the performance and integrity of pressure gauges from harsh conditions such as pulsation, vibration, pressure spikes, clogging and/or corrosion.
These accessories may also help with other pressure measuring instruments such as pressure switches and transducers.
Flow Restrictors
Pulsation can damage equipment or cause inaccurate readings. In processing equipment, surges in process pressure can lead to pulsation in gauges. These pulsations can cause poor gauge readability due to pointer flutter and decreased gauge integrity due to component damage.
Pressure snubbers and pulsation dampeners are used to lessen the effects of pulsation. These accessories dissipate pulsation before it reaches the instrument by restricting the flow of the process material.
Other flow restrictors such as needle valves and throttle screws can change the size of the opening on a pressure instrument to combat pulsation. Some accessories are more adjustable than others.
Needle valves can be opened wide or closed to control flow, much like a spigot for a garden hose. Throttle screws thread into the communication hole of a pressure gauge and shrink it down from a standard hole to a pinhole, which effectively controls the impacts of pulsation.
A pressure surge can cause a gauge to spike beyond the maximum allowable working pressure, which will likely damage the instrument beyond repair. Pressure limiting valves close once the pressure reaches a set point and only reopen when it’s safe again to protect the gauge.
For example, if you had a pressure gauge installed on a system that typically operated at 10 psi, you would ideally be using a 0-30 psi range. If the system could potentially experience higher pressures (over 38 psi), either during a clean down process or a pressure test, it could damage the gauge.
In this instance, a pressure limiting valve could be set to close at 30 psi and the gauge would remain protected until the pressure dropped to a safe level.
Pressure switches can also be assisted by the use of a pressure limiting valve. Pressure switches are mechanical devices and can be limited to percentages of span on where the setpoints can be.
For example, a 0-100 psi switch can have set points between 15 and 90 psi. If you have a system that operates at 80 psi, you will want a switch with a span of 0-100 psi (or higher). If you need a 5 psi decreasing set point, your switch may not be able to accommodate because you’re limited to 15-90 psi.
One strategy to resolve this is with a pressure limiting valve. You could use a switch with a span of 0-30 psi in conjunction with a pressure limiting valve set at 30 psi. This would allow you to operate safely at 80 psi (in which case the pressure limiting valve would be closed protecting the switch).
When the pressure decreases below 30 psi, the pressure limiting valve opens allowing the switch to operate. And a 0-30 psi switch can achieve the 5 psi decreasing set point.
Extreme Temperature Protection
Instruments also need to be protected against extreme temperatures. For example, coil siphons are designed for steam applications. They help protect gauges from high-temperature steam by creating a condensate barrier and isolating the instrument from the steam.
In other extreme temperatures applications, a capillary can help keep instruments safe from damage. One part of the capillary connects to the process and the other connects to the gauge allowing you to mount the gauge somewhere safe away from the extreme temperatures of the process.
Capillaries also help reduce the temperature by reducing the volume of the media passing through and allowing the ambient temperature to cool (or heat) the media.
However, capillaries can get clogged easily. Because capillaries have a small communication hole they also act as effective flow restrictors and can help with pulsation. They also provide flexibility with where instruments are mounted, so where there is heavy vibration an instrument can be relocated to a more stable environment.
In applications that would be prone to clogging you may need to protect the capillary with an isolation device known as a diaphragm seal or isolation ring.
Protection from Clogging and Corrosion
Diaphragm seals and isolation rings are designed to protect instrumentation from clogging and or corrosion. They are installed between the instrument (as well as associated accessories) and the process.
They use the displacement of a transfer fluid (caused by the deflection of a thin diaphragm or liner) to transmit process pressure to the instrument.
They can be constructed from many different types of materials for applications that use harsh chemicals, or with larger openings and more robust sensing elements to protect against particulates in the process.
Can Instrument Accessories Impact Accuracy?
Now that you have a rundown of the various pressure instrument mounting accessories and what they’re used for, do any of them impact the accuracy of your measurements?
The short answer is that most of them do not.
Flow restrictors such as pulsation dampeners, snubbers, throttle screws, needle valves and especially capillary would likely impact the response time on registering a pressure reading, but it would not impact the accuracy of the instrument.
The more flow restriction applied, the longer the response time you could expect. But ultimately, slowing down the response time is the desired result of using a flow restrictor. By slowing the response time, you are effectively minimizing the impact of pressure pulsations.
Diaphragm seals, however, can impact accuracy. The seals require additional pressure to displace their fluid, which can affect the accuracy of your pressure measurement.
The displacement and spring rate of the diaphragm must be enough to actuate the sensing mechanism of the instrument. Because the diaphragm has a finite spring rate, it will require some pressure to displace the fluid necessary to operate the pressure element.
This impact on accuracy will be greater on instruments with elements requiring more fluid displacement, with diaphragms having higher spring rates, and on lower pressure range instruments. It is a best practice to recalibrate the instrument with the seal attached.
Liquid-filled diaphragm seals may also create a detectable error in your pressure measurement due to temperature fluctuations causing expansion or contraction of the fill fluid. You would need to calibrate the seal and instrument at the process service and ambient temperature to ensure accuracy, as well as insulate any components exposed to outside temperature/elements.
Reliable, high-quality pressure instruments built to ASME code specifications will help ensure that you’re getting the most accurate readings possible from your pressure measurements.
Ashcroft’s pressure instrument assembly guide is a good selection of quality instrument mounting products to help keep your pressure measuring instruments running efficiently, safely and accurately without fear of equipment failure.
We don’t like to pressure you, but we have more information.
You should now have a better understanding of what pressure instrument accessories can be used for and whether they have an impact on the accuracy of your pressure measurements.
You can always contact us with any questions or concerns. Our industry experts at Ashcroft can answer your questions and help you get back on track!
If you want more information on these types of mounting accessories and assemblies, view our webinar about pressure instrument mounting assemblies.
You can also view some related articles we've written:
- What’s the Right Pressure Instrument Mounting Assembly for Me?
- When to Use a Welded Instrument Assembly
- Why Do I Need a Steam Siphon and How Does It Work?
- Selecting the Right Manifold Valves for Your Pressure Gauge
- How Does Media Temperature Affect Pressure Transducer Performance?
Or download our guide to pressure instrument assemblies: 