Best Practices for Installing Flanged Diaphragm Seals

This article was originally published on January 31, 2022 by John Varkonda and was updated on May 8, 2024, by Todd Thomas. 

Keeping pressure measurement instruments from becoming damaged in harsh environments is a challenge in many industries, including water/wastewater. Diaphragm seals play a vital role in these applications by creating a leak-tight and corrosion-resistant barrier between a pressure instrument and your process. 

For many, a flanged diaphragm seal connection is a great choice because it provides a convenient way to attach instrumentation directly onto flanged process piping without having to use threaded connections. But proper installation of these seals is the key to getting the most protection. 

Ashcroft is a recognized global leader in pressure and temperature instrumentation and uses our decades of experience to educate and guide customers to the solutions that best meet the challenge at hand. In this article, you will learn a bit about flanged diaphragm seals, the basic principles of torque and best practices for installing a flanged diaphragm seal in your process. 

When you are done reading, we will also share additional resources that may be beneficial to you.  

About Flanged Diaphragm Seals

Several standards govern the dimensions and pressure ratings of flanged diaphragm seal connections. In the U.S., ASME B16.5 flanges are the most common, while EN or API standards are also used in other areas.

You may find overlap in nominal sizes for these different types of flanges, however in most cases, the bolting pattern, thicknesses and overall pressure ratings differ substantially. For this reason, you cannot mix and match different flanges at the same joint. Other than that, installation and operation are similar regardless of the type of flange.

Flanged diaphragm seals are supplied with the necessary bolt circle, thickness, size and facing to mate up with the specified mating flange. While raised-face flanges are the most common, they are also available with ring-joint, flat-face, or other flange facings as required by the pipe specification or application.

For most seals, the supplied flange comes as a separate component that slides over the diaphragm housings. Seals with flange sizes where the bolt circles might interfere with the diaphragm diameter may be supplied with a lower housing machined with the proper flange form and dimensions and are often supplied with threaded studs already installed for convenience.

Figure 1. Ashcroft® Flanged Diaphragm Seals.

Other seals, such as the Ashcroft® DF Flush Flanged Diaphragm Seal may have a welded flush diaphragm to the face of the mating flange.

Figure 2. Example of Flush Welded Diaphragm Seal. 

For safety, it is important to select a flange with a pressure rating that exceeds the anticipated process pressure and the indicating range of any instrument that is attached. Elevated process temperatures lower the pressure rating of a given flange type, and different flange materials may have different pressure ratings.

The chart below illustrates the maximum allowable pressures for carbon steel and stainless steel flanged diaphragm seals. This pressure is determined by the flange material, the pressure class of the flange and the temperature to which the flange will be exposed. The full-scale range of the instrument should not exceed the maximum allowable pressure (psi) of the flange as shown below.

Figure 3. Flanged Diaphragm Seal Maximum Allowable Pressure Chart.

What you need to know about torque.

When installing flanged diaphragm seals, you must consider torque.

Torque is the measure of force causing an object to rotate:

• Too much torque can cause the flange to deflect or damage process piping – especially elastomeric or glass-lined piping.
• Too little torque can result in a bad seal and the potential for leaking process fluid.

We are occasionally asked what our recommendations are for bolting torque on our seals. Generally speaking, we aren’t able to provide torque recommendations, as most of the factors that determine a target bolt torque are outside the scope of what we provide.

The ASME PCC-1 specification describes the process for determining appropriate bolt torque. The calculation relies on several installation assumptions outside of Ashcroft’s control. For example, the use of bolt lubricant can reduce needed torque by 20% or more compared to dry bolts.

Additionally, different gaskets will have varying minimum and maximum sealing stresses. However, the bolting technique also has a significant impact on flange installation. According to ASME, “consistency of applied torque and load … is usually much more important to the success of the joint than achieving a particular pinpoint torque value.”

How to install a flanged diaphragm seal Installation. 

Before installing a diaphragm seal, be sure to select the appropriate nuts, bolts, washers, gasket and thread lubricant. Make sure that the gasket is made from an appropriate material for the process media and conditions. Gaskets should also be selected according to the type of flange face used.

To ensure a leak-tight connection between the mating flange and the flanged diaphragm seal, we recommend taking the following steps:

After at least four hours, the bolts should be tightened one final time to the target torque value to account for creep/relaxation of the bolts.

Special considerations for torque.

ASME specifications will generally cover metallic flange applications. In cases where alternate materials are used, such as elastomeric or polymeric flanges, it is important to consider that less torque may be required to form a pressure-tight seal than with metallic flanges and/or gaskets. In these scenarios, a qualified piping engineer must determine the appropriate torque for a given installation.

Ready to learn more?

Now that you have a better understanding of how flanged diaphragm seals are installed, the considerations of torque and the effect it can have on your process, you may want to gather more information on the subject. The following resources may be of interest: 

• Pressure Instrument Mounting Assemblies (webinar recording)
• What’s the Right Pressure Instrument Mounting Assembly for Me?
• Isolation Ring Assemblies for the Water/Wastewater Industry that are Accurate and Reliable

You can also contact us directly. Our industry experts can help answer your questions. In the meantime, feel free to download our Complete Guide to Pressure Instrument Assembly.