Pressure Transmitters: Improving Diaphragm Seal Maintenance

Summary

Diaphragm seal pressure transmitters are commonplace across industry. Whether they consist of an integral or remote seal configuration, they offer a reliable way to measure pressure and level in a wide variety of applications. While the specification of proper diaphragm seals and capillaries is well documented, flushing/calibration ring selection is often less understood.  

This article discusses what flushing rings are, outlines the latest design improvements, and details benefits when they are used in level measurement systems, which is the most common application. 

A flushing ring is positioned between the diaphragm seal and a tank connection, as shown at the top of this article. These rings can be made of a variety of materials and typically have one or more process connection ports, each serving a variety of functions.  

If plugging and/or material build up is a problem in the tank nozzle, the ports on the flushing ring can be used to inject air, water, or solvent to clean the face of the diaphragm seal and clear any deposits that may be forming. This improves reliability and extends the time between tank maintenance outages. In particularly difficult applications, the flush ports can be connected to a solenoid, which activates intermittently to inject an appropriate fluid to keep the tank nozzle clear.  

Flushing ring features 

Different styles of flushing rings are available with varying price points and capabilities. The standard type shown at the top of the article has one or two threaded and plugged ports and is bolted between the diaphragm seal and the tank. Nipples and small isolation valves can be added to these ports to facilitate draining and venting.  

While less expensive initially, standard types of flushing rings require the user to specify the correct piping and valve components, and ensure the materials, O-rings, and pressure and temperature ratings are appropriate for the application. They also require on-site assembly and have multiple potential leak points due to the number of required fittings. 

An alternative is a prefabricated, bolt-through flushing ring with integral valves (Figure 1). These types of flushing rings are fire-safe and come in a wide variety of flange sizes, with many wetted material options. Needle, ball, and gate valve options with assorted connection types and materials of construction are available to suit most applications.  

A common installation will have a vent and drain valve for calibration applications, or it may just have a single valve for flushing applications. This flushing ring assembly comes prefabricated and leak-tested, so installation is quick and easy. It also has fewer potential leak points than standard flushing rings, and it can be stocked as a single assembly for quick change out. 

Flushing ring alternatives 

Recently new compact flushing ring designs have been introduced (Figure 2). These have all the features and benefits of a standard ring assembly but add tangential injection of the process fluid into the ring.  

Tangential fluid injection creates a vortex cleaning action that removes residue build-up up to five times faster than a standard design. It also can clean up to 30% more diaphragm seal surface area, providing a significant advantage in applications that are difficult to clean. (See the operation of a compact flushing ring compared to a standard design in this video.)  

The compact, integral valve design of this new type of flushing ring lets it fit in much tighter spaces where a traditional flushing ring might not fit. 

Flushing rings in action 

One main differential pressure application is measuring the level of process media in a tank. If the tank is vented, the transmitter is typically installed at its base to detect head pressure. This pressure reading is converted to level based on the specific gravity of the tank contents using the following formula:   

Tank Level (inches) =  
Head Pressure (inches of water column) / Specific Gravity of liquid 

If the tank is closed and under variable pressure, a second diaphragm seal is added at the top of the tank to detect the tank vapor pressure. This second seal is connected to the low side of the differential pressure transmitter, allowing the device to measure the difference of the two readings to determine the resulting head pressure, which is converted to tank level using the same method discussed previously (Figure 3). 

Flushing rings are sometimes referred to as calibration rings because they enable easy calibration of the level transmitter without taking the tank out of service. Calibration of a differential pressure transmitter requires the upper seal to be vented and removed from pressure, while the lower seal is subjected to a known calibration pressure to span and zero the transmitter.  

If no flushing/calibration ring is installed, an in-service transmitter calibration requires removal of the entire assembly to clean the diaphragm seal. This procedure takes an extended amount of time, during which the process is offline. Diaphragm seals can be quite heavy, so between the mechanical effort involved, as well as the potential for hazardous material exposure during the flange breaks, calibrating a diaphragm level transmitter while the tank remains in service can be quite challenging, and potentially dangerous.

The installation of flushing and/or calibration rings with integrated vent/drain valves makes calibration much simpler. Flushing/calibration rings also make the flushing procedure significantly safer and faster, and it avoids potential diaphragm seal damage, which can easily occur when a seal is unbolted and bolted back into place.  

For these reasons, flushing/calibration rings and tank isolation valves are strongly recommended whenever a tank cannot easily be removed from service, and in other situations, such as when sludge or debris is present in the process media. 

Conclusion 

When caught up in the detailed specification of diaphragm seals and differential pressure instruments, it is easy to forget flushing rings. However, this component is often crucial to maintain reliable performance, enable easy and safe in-service maintenance of the seals and the instrument, and ease calibration. 

If a flushing ring does make sense for the application, it is wise to evaluate the various options that are now available, particularly compact designs with faster and more thorough cleaning capabilities. Particularly in a large project with a wide variety of transmitters, prefabricated and factory-tested flushing rings will provide significant economic and performance advantages as compared to traditional designs. 

As is often the case, the best designs require attention to detail at every stage and for each component, including flushing rings. The result will be many years of trouble-free service, with easier calibration and improved safety. 

<sub>All figures courtesy of Emerson

This feature originally appeared in the October 2023 issue of InTech digital magazine.</sub>