Is the venting function, and potential hazards, easily overlooked?
Preamble: This article is intended primarily to inform users of existing and/or planned installations which use balanced bellows spring operated pressure relief valves (PRV’s). The article is further written not do discourage use of balanced bellows PRV’s nor to steer the user away from their design. Above all, Pressure-Relief Valves, also called Safety Valves, Relief Valves or simply Safety/Relief Valves, are end of line devices providing the last line of defence for life and property against system overpressure events. Their use should not be compromised regardless of their design. Balanced bellows PRV exist and remain a popular economical selection. The topic herein is to alert potential hazards which, in the authors experience, are vastly unknown, overlooked and too often, ignored.
Spring operated Pressure-relief Valves (PRV’s) fitted with balancing bellows (and/or balancing piston), are widely used primarily to negate the effects of variable back pressures which would affect the predetermined set pressure of a PRV by increasing it by an unsafe additional value.
The bellows and its attached body/bonnet held flange-like extension, prevents any back pressure to act against the area at the back of the disc. The disc is attached to a holder as part of a bellows assembly (P1 below). The bellows has roughly the same area as the seating area of the disc, and balances out the effects of any back pressure since pressure can no longer push against the back of the disc.
P1 The yellow outline in the picture above, represents the bellows and its extension held between the body and bonnet, in turn attached to a disc holder forming an assembly. All parts above the bellows yellow outline, are exposed to atmospheric pressure only.
For the bellows to work and balance out any back pressure, the non-back pressure affected side of the bellows assembly ie. the bonnet chamber (which houses the spring, spindle sometimes guide etc.), is kept within an atmospheric pressure environment, which is maintained by the open Bonnet Vent (as shown in P2 below).
API Standard 520 Part 1 states “The internal area in a balanced-bellows spring loaded PRV is referenced to atmospheric pressure in the valve bonnet. It is important that the bonnet of a balanced PRV be vented to the atmosphere for the bellows to perform properly. If the valve is located where atmospheric venting would present a hazard or is not permitted by regulations, the vent shall be piped to a safe location that is free of back pressure that may affect the PRV opening pressure”.
P2 Typical balanced bellows PRV with bonnet opened to atmosphere.
The Bonnet Vent, which is a nominal ½” to ¾” NPT(F) bore, depending on PRV size and/or manufacturer, MUST not be closed from atmospheric pressure at any time.
Note. Manufacturers generally utilise the same bonnet for both conventional and bellows type PRV’s. During assembly, the bonnet designated as being required for a conventional PRV will have the bonnet vent plugged.
All clear? But what are the Hazards?
Some things you should be aware of…a Discussion.
API-520 Part 1 Goes on further to quote “It is important to remember that the bonnet of a balanced PRV shall be vented to atmosphere at all times. The user should be cautioned of the potential for freezing of atmospheric moisture inside the bonnet in cold service due to autorefrigeration or cold ambient temperature”.
There are some other common hazards, involving the bonnet vent that are not properly understood...
Let’s say the bellows fails in service for whatever reason, and the balancing effect is lost. The PRV will now open at a pressure above the predetermined set pressure. Most probably above the MAWP. This is a potentially unsafe situation.
End users and specifiers have identified situations where bellows can fail and where the set pressure MUST be maintained without any increase. An option offered by the majority of PRV manufacturer’s is the use of a Balanced Piston type PRV (example P3 below). Here in addition to the bellows, the PRV incorporates into its construction, a balanced piston with its guided area usually made as a close-fitting labyrinth or seal. The cross section of this piston is also equal to the disc seating area and acts as an Auxiliary or Supplementary balancing component for a failed bellows in service. Set pressure is maintained in event of bellows failure.
Unless the API-520 recommendation of piping away the bonnet vent to a safe, atmospheric location is known about and properly followed, there will occur one serious, potentially harmful issue.
In case of Bellows Failure…There will be constant flow out of the Bonnet Vent generated by back pressure (and possibly from other sources if from a common header downstream).
If the escaping fluid is toxic and hazardous, then the area around the PRV installation is quite possibly and unknowingly a serious risk to human life.
API-Standard-520 Part 2 Provides the user/operator with recommendations for piping away non-hazardous and hazardous process gas/vapour and liquid fluids from PRV Bonnet Vents.
This is the minimum Risk Analysis all users should undertake.
The escape of fluid is also true of the balanced piston design should the piston seal be compromised. A balanced piston design is to be purely considered as a temporary fix only to maintain a predetermined and critical set point. Hence the optional balanced piston is an auxiliary or supplementary feature to the bellows. In a different time, this arrangement used to be called the ‘Belt & Braces’ design. The balancing piston with its seal will not completely prevent the loss of any escaping fluid over time.
Note that some manufacturers have specific PRV designs with balancing pistons but without bellows, which are used for particular and/or known applications.
A small number of manufacturers provide their balanced bellows PRV’s with the feature of a ‘Bug Screen’ (see P4. below), as a standard feature, though the option should be available from all vendors or even fitted on site by users. Bug Screens must still have to vent to atmosphere.
P4. Example of Bug Screen fitted to balanced PRV bonnet vent.
Such a feature prevents the ingress of dirt, sand and insects to collect inside the PRV. These are typically 90° elbows with some form of mesh or screen inside and have the exit pointing downwards. But they must be able to maintain unobstructed atmospheric pressure to the bonnet chamber.
Apart from the few manufacturers providing bug screens, all other manufacturers ship newly made bellows PRV’s with plastic plugs, often in a bright colour, press fitted into the bonnet vent to prevent ingress of dirt during shipment. Instructions are not always followed, and these plugs can remain in bonnet vents long after installation.
Following is an example of an installation with such push-in plugs, in red, left in (P5.).
P5. Example of shipping plugs left in the bonnet vent.
It is unlikely that the set pressure will be affected in this example, since should the PRV’s open, the plug will fly out at high speed – itself a hazard. But if care is not recognised at this point what hope for the unseen?
The point I keep on making in the above discussions, is that there can be some serious issue if the risks of bonnet venting, is not properly understood at site by users.
Some of the real hazards to human life can be easily overlooked. Certainly, in the event of bellows failure, unless the operators ground crew are well experienced and knowledgeable, no one will even notice that a bellows has ruptured and is venting a hazardous substance in the immediate area, unless it is too late.
UK Government Health & Safety Executive (HSE) Was notified of issues regarding bellows PRV Bonnet Vents back in 2002 and ordered an immediate review and potential shutdown of an installation with such malpractices. Their finding included discovering that bellows PRV’s with leaking bonnet vents simply had a metallic plug installed to stop the constant leak and finding that some bonnet vents had one-way ‘tell-tale gauge’ arrangements to monitor any bonnet vent leakage. Unbelievably, these were fitted with one-way check valves which would block the vent once a bellows ruptured (!!) – These examples were found on an offshore platform processing hydrocarbon fluid. A ‘Darwin Award’ if there ever was one!
How far this was taken I do not know. But the HSE certainly considered it a dangerous practice and against all safety regulations possibly teetering on prosecution. You can read an edited version of that report right here,’
Isn’t it about time some viable Bonnet Vent Sensing Device was available to alert the user/operator immediately on failed bellows?
I also strongly feel that PRV manufacturers consider attaching a small but visible tag close to the vent, with an internationally recognisable symbol and words as to not plug or block the Bonnet Vent?
To end this article, I would be recommending that with the new and usable technology we now have available, Hazardous PRV Bonnet Venting be better addressed. Particularly with the availability of Remote & Wireless PRV Monitoring using the various ‘bolt-on’ instrumentation sniffers & sensors boasted by the major PRV manufacturers.
I hope that you have been informed more on the simple, but important issue on PRV bonnet venting.
If interested, you can find other PRV articles of mine here on LinkedIn covering PRV API (effective area) Vs ASME (actual area) sizing, and a type of a guide to using API-Standard-526 against ASME VIII requirements
