When would I use a dampening device with a diaphragm seal?

A dampening device, such as a pressure snubber or pulsation dampener, may be recommended with a diaphragm seal when pressure pulsation, surges, or spikes in the process could damage the pressure instrument or make its readings difficult to interpret.

In challenging process applications, diaphragm seals, also called isolators, protect pressure instruments from corrosive media, abrasive substances, and large particulates. However, the assembly may need additional protection when the application also involves severe pressure fluctuations. In these cases, a dampening device can restrict these fluctuations before they reach the instrument to help stabilize readings, reduce wear on internal components, and extend instrument life.

Read this article to learn why pulsation control may be needed with a diaphragm seal, how different devices work, and what to consider when selecting one.

How does a dampening device protect a pressure instrument?

Dampening devices restrict rapid pressure changes before they reach the instrument.

Pressure pulsation may occur as low-frequency, high-amplitude surges (long, continuous strokes), or as high-frequency, low-amplitude surges (short, rapid strokes). Exposing a gauge to these pressure fluctuations can cause:

• Poor or unstable readings
• Significant wear on internal components
• Reduced instrument life
• Complete loss of instrument function

In one extreme example, pulsation damaged a gauge so severely that metal shavings were projected through the pinhole in the gauge case and dial.

Pressure snubbers and pulsation dampeners address fluctuations transmitted through the process media. Mechanical vibration traveling through equipment, piping, or the mounting point may require other protective measures, such as liquid filling, a dampened gauge movement, or remote mounting.

What applications may need a diaphragm seal and pulsation control?

There are many applications where both a diaphragm seal and dampening device may be needed to protect instruments and the process. A few examples include:

• Pumping systems 

• System integrators, skid builders, and pump manufacturers

• Chemical plants, water treatment facilities, and refineries 

Because pumps and valves can produce pulsation, surges, and spikes, selecting the appropriate protection requires an understanding of the process media, pressure behavior, instrument configuration, and required response time.

What types of pulsation-control devices can be used with diaphragm seals?

Several pressure snubbers, pulsation dampeners, and restriction devices are available for different process conditions and measurement requirements. Here's a closer look at how they work: 

A sintered or filtered pressure snubber slows pressure movement through a porous metal disc.

The Ashcroft® 1112 Pressure Snubber disc acts as a controlled restriction, but this filtering action also slows the instrument’s response to pressure changes. When using this design with a diaphragm seal, make sure the porosity is appropriate for the fill fluid. Some porosities are designed for clean air or water and may be too restrictive for oil-based fluids.

Figure 1: Ashcroft 1112 Pressure Snubber

A piston-style dampener restricts pulsation using a moving piston and a selected pinhole.

The Ashcroft® 1106 Pulsation Dampener includes five pinhole sizes so the restriction can be matched to the application. As pressure changes, the piston moves within its chamber and helps keep the pathway clear of particulates. This self-cleaning design can be installed vertically or horizontally.

Figure 2: Ashcroft 1106 Pulsation Dampener

An adjustable pressure snubber can be opened or tightened to provide the required amount of pulsation control.

A design such as the Ashcroft® PD02 Pressure Snubber can be opened while the diaphragm seal assembly is filled and calibrated, then tightened to establish the required restriction.

Adjusting the snubber changes the system’s internal volume, which can affect accuracy at smaller pressure spans. If the instrument provides zero and span adjustments, this effect may be corrected during calibration. Otherwise, it must be considered when accurate measurements or pressure-switch setpoints are involved.

Figure 3: Ashcroft PD02 Pressure Snubber

Throttle valves and throttle screws can also restrict pulsation, but they may create challenges when used with diaphragm seal assemblies. Like the pressure snubber, a throttle valve can be opened during filling and then adjusted to control pressure fluctuations. However, each adjustment changes the assembly’s internal volume and may affect accuracy, response time, or switch setpoints. For this reason, its effect on the complete system must be evaluated.

Figure 4: Ashcroft 7001 Throttle Valve

Throttle screws and plugs are commonly installed in gauge pressure connections to minimize pulsation. Ashcroft typically uses them with low-pressure gauge ranges, liquid-filled gauges, and gauges with PLUS!™ Performance.

However, the small opening can prevent the fill fluid from pushing trapped air out of a diaphragm seal assembly. The throttle screw is therefore generally omitted. When pressure fluctuations are expected, an external snubber or dampener may be needed instead.

Why is pulsation control important when an assembly includes a gauge and switch?

Added pulsation control can help protect both instruments when a pressure gauge and pressure switch share the same diaphragm seal assembly.

The gauge may have a dampened movement or liquid-filled case that helps stabilize its pointer. However, these features do not protect the pressure switch. Placing a pressure snubber or pulsation dampener ahead of both instruments reduces the fluctuations entering the entire assembly.

Figure 5: Diaphragm seal assembly with pressure gauge, pressure switch, and pulsation-control device.

How do you select the right dampening device for a diaphragm seal assembly?

The right solution must reduce harmful pressure fluctuations without slowing the instrument’s response more than the application can tolerate. Too little restriction may allow damaging pulsation to reach the instrument. Too much can prevent the instrument from responding quickly enough to legitimate pressure changes.

Consider the following factors:

• Pressure behavior: Determine the frequency and amplitude of the pulsation and whether the system experiences surges or spikes.
• Required response time: Consider how quickly the instrument must respond to a change in process pressure.
• Fill fluid: Confirm that the restriction or porosity is appropriate for the diaphragm seal fill fluid.
• Pressure range: Consider whether changes in internal volume could affect accuracy at smaller pressure spans.
• Instrument configuration: Account for every instrument connected to the assembly, particularly when a gauge and switch share the same diaphragm seal.

The diaphragm seal, fill fluid, selected restriction, pressure range, and attached instruments should be evaluated as one complete measurement system.

Learn more about protecting diaphragm seal assemblies

A diaphragm seal protects a pressure instrument from challenging process media, while a pressure snubber or pulsation dampener reduces the effects of pressure pulsation, surges, and spikes. Applications that combine these conditions may need both forms of protection.

Check out the related resources below or Contact us if you need help determining whether your diaphragm seal assembly requires additional pulsation protection.

In the meantime, download our guide to find the right instrument assembly for your specific needs.