MORE TOPICS | Pressure Gauges & Switches
Protecting Pressure Instruments in Suction & Discharge Installations
Recommendations for keeping pressure gauges, switches and sensors working properly.
John Girard | Ashcroft
When working in a chemical manufacturing facility, water treatment center or oil/gas refinery, the use of pump systems to transfer products between different stations is likely. Suction and discharge installations can be particularly difficult environments for the instruments used to monitor the pressure in these systems. If instruments get damaged, the entire process can break down, so protecting them is a priority.
Pressure gauges, switches and sensors are commonly found on both the suction and discharge sides of pump systems. They may be directly connected to the process or isolated using diaphragm seals or isolation rings. Pumps often cycle on and off. This is typically an automated process that can lead to pressure transients with the potential to damage pressure gauges, switches and sensors.
Sudden surges of pressure when pumps start can exceed the proof pressure of instrumentation, while the shutdown momentum can cause a water hammer effect, resulting in pressure spikes that may damage unprotected devices.
Although these pressure transients are harmful and can initially be hard to predict or detect, they are not the biggest problem associated with pump systems. Pump systems are notorious for creating both vibration and pulsation in instruments close to the pump.
Addressing Instrument Challenges
For mechanical pressure gauges
There are several ways to protect mechanical pressure gauges in harsh conditions associated with pump systems. Following these strategies can help ensure pressure gauge effectiveness and extend the life of these instruments.
| IMAGE 1: Diaphragm seal assembly (Image courtesy of Ashcroft)
1. Install American Society of Mechanical Engineers (ASME) B40.100 recommended protection devices directly to the process.
- Pressure snubbers and pulsation dampeners protect gauges by absorbing sudden changes in pressure that commonly occur in pump systems. Using these devices can help ensure the accuracy and reliability of pressure instruments and extend their lifespan.
- Pressure-limiting valves prevent pressure from exceeding a predetermined set point and will only reopen once the pressure drops below a certain value. This helps create a more stable and controlled operating environment.
- Capillaries can be used in severe cases when instruments need to be removed from direct contact with the installation point to protect them from vibration, pulsation and elevated process temperature.
2. Use a liquid-filled gauge case.
An alternative to a liquid gauge is to use a pressure gauge engineered with an internal dampening media. These types of gauges offer the same effect as a liquid-filled gauge but without the risk of leaks.
3. Use a dry gauge with an internal dampening mechanism.
Gauge cases that are filled with glycerin, silicon or halocarbon are also helpful in dampening the effects of pulsation and vibration in these applications. They also keep moisture out of the case and condensation off the window while lubricating the internal mechanical components of the gauge. However, keep in mind that liquid-filled gauges also have a risk of leaking due to high vibration or excessive change in temperature, especially if installed outside.
4. Add throttle screws or plugs into the pressure connection.
Whether it is a dry gauge, liquid-filled gauge or a gauge with dampening media, throttle screws are recommended to help manage pressure pulsations. These devices are inserted in the orifice of the pressure connection to reduce the orifice size and minimize the impact of pulsations.
5. Keep the pressure gauge span between 25% and 75% of full scale.
When selecting the span for a pressure gauge, ASME B40.100 guidelines recommend keeping the normal operating pressure between 25% and 75% of full scale. While this remains consistent across applications, leaning toward the 25% mark can be beneficial. Opting for a larger range accommodates pressure transients and also helps minimize the effects of pulsation and vibration. A smaller pressure span can leave mechanical pressure gauges more susceptible to these challenging conditions
For mechanical pressure switches
With mechanical pressure switches, it is harder to notice the effects of pulsation and vibration. Although these are industrial products, they are still carefully calibrated instruments that can be impacted by these conditions.
The effect of pulsation and vibration can cause problems with longevity and/or repeatability. This is why pulsation dampeners and pressure snubbers help protect these instruments. In cases of extreme vibration, pressure switches can be remotely mounted using capillaries.
For diaphragm seal assemblies
As previously mentioned, all instrument protection devices can be installed directly into the process or isolated using a diaphragm seal or isolation ring. These isolators serve to protect the instruments from various hazardous conditions such as clogging, corrosion or extreme temperatures. While diaphragm seals do offer some dampening, it is often recommended to include additional dampeners or snubbers to protect the instruments effectively.
Assemblies can accommodate at least one instrument, and sometimes several. However, if there are multiple instruments, all instruments must have the same range, which is the widest range possible while maintaining the normal operating range within 25% to 75% of the dial (for pressure gauges). A broader range provides better protection, and when installed on a diaphragm seal, they exhibit better responsiveness to pressure fluctuations.
Note: Low-pressure systems may experience sluggishness readings when isolated with a diaphragm seal. Ultimately, the instrument’s performance depends on the pressure exerted on the isolator’s sensing element to displace enough fill fluid to operate the instruments, which can be challenging with lower pressures.
Digital Pressure Gauges & Sensors (Transducers & Transmitters)
Digital pressure gauges and pressure sensors can be much more forgiving under challenging circumstances. These instruments are engineered to handle overpressure conditions, and the effects of vibration are not as detrimental as they are on mechanical products. Often, sensor-based instruments can slow down the reaction time to minimize the effects of pressure pulsations.
John Girard is a business development leader at Ashcroft. For more information, visit ashcroft.com.
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