Types of Pressure Relief Valves

When asked about the kind of relief valve required for a piping line, three suppliers will provide differing answers. Yet these different Types can be qualified because at least three different things are meant by the term "type": namely, how the valve is actuated, how it is constructed, and what it is protecting. For example, a "balanced bellows sanitary relief valve" gives answers to all three in one stroke; hence, the term sounds too inclusive while at the same time being insufficient.

Now that you have understood the three Types, proceed to form questions in the appropriate order; the first thing to inquire is the backpressure. It is indeed the only variable determined by the piping itself prior to making a purchase, and it turns out to be the most expensive to change later on. If you get it wrong, then you will need to replace a discharge header instead of the spring.

Quick answer
Backpressure is what picks the family
Conventional spring-loaded
Balanced bellows
Pilot-operated
Construction: direct-acting, piston, diaphragm
Safety valve, relief valve, safety relief valve
Hygienic and sanitary designs
Vacuum relief
Rupture discs
Material follows the fluid
Installation mistakes that undo a good selection
How to choose, in order
Brands and what they cost
FAQ
References
Conclusion

Quick answer

To put it simply: standard spring-loaded valves can withstand backpressure under 10%, balanced bellows valves from 10% to 30%, and pilot-operated valves above that level. Next up is the deciding mode of action — direct, piston or diaphragm — with the last being the choice if cleaning is a must. Next comes the materials and here choice is determined by type of fluid and not the pressure of the fluid.

Backpressure Is What Picks the Family

There are two types of backpressure and purchasers get confused by the two terms.

Superimposed Backpressure is present in the discharge header when your valve opens, created by other valves upstream. The built-up Backpressure occurs only after your valve begins to act and is due to the resistance of the discharge against the smaller pipe size of downstream piping. The first is incurred while the other needs to be avoided by ensuring that outlet branches are designed to take into account the flow requirement rather than the valve size.

Both types push against the disc of the valve, influence the set point, reduce the capacity and cause chattering that can ruin the valve seat in a few hours.

Total backpressure, % of set pressure	Family that works
0–10%	Conventional spring-loaded
10–50%	Balanced bellows
50% up to roughly 90%	Pilot-operated

There is nothing random about the numbers. API 520 Part II has placed a specific limitation on the other side of the valve, too: the drop in pressure between the vessel and valve inlet at rated capacity should not exceed 3% of the preset pressure. That is the inlet rule that generally gets forgotten in the heated discussions about the outlet. A valve that is positioned at the end of 40 feet of incorrectly sized piping will start chattering before the header comes into play.

There are two limits that are in conflict with or even overpower it. The first one is the pressure of the actual working process and set pressure. A working process running at 92% of set pressure will keep bubbling at a normal valve despite the outlet piping system, while backpressure calculations will not help the system. The second limit is the capacity, since the relief load is large and prompts using a bigger valve, which definition constantly changes.

It is also important to make this distinction between the check valve now because the devices are similar enough. The check valve prevents the backflow of the liquid. However, it does not reduce pressure in a closed system, nor does it serve as overpressure relief. I once stood near a positive displacement pump, where the check valve was the only protection device, and the argument was that it “keeps the pressure inside.” It holds it in until the pump deadheads against it. The comparison is set out in this explanation of whether a check valve counts as a pressure relief valve.

Conventional Spring-Loaded

In terms of operation, this design is the simplest among those in the catalog. A spring mechanism keeps the disc pressed against the port. The force exerted by the inlet pressure on the surface of the port is able to overcome the spring force, allowing the lift of the disc and discharge of the fluid. When the inlet pressure falls, the spring mechanism pushes the disc back into position. There are no parts that can break except for the spring and the seat.

This is why the design keeps the title of the most utilized construction around the globe and the most economical at the same time. A half-inch brass valve for any type of substance, be it air or water, will range from $60 to $150 depending on whether it is certified or not. The Crosby JOS-E is somewhat more expensive, but it is the same construction made of better materials.

The only limitation is this – the bonnet is vented on the outlet side, so the backpressure directly impacts the disc. Once again, there are three things that the backpressure will influence: opening and closing cpactus pressures. It should be understood that above approximately 10% of the adjusted gas pressure, the valve will malfunction because of the wrong spring, so the valve will operate late or inappropriately.

The correct application would be when this type of valve is used in the arrangement where it discharges gases into the atmosphere and where there are no other lines connected to it.

Balanced Bellows

Wrap the bellows around the stem, after which the spring chamber cannot witness the outlet space anymore. Hence, the backpressure does not affect the working disc surface, and the setting point stays at the already set position.

The working range is between 10 and 50% of the setting pressure, which is the standard possible value for most headers. The LESER 526 is the clear example from Europe, however, practically all the big factories have its analogues on the market. When deciding on the bellows to use for your particular case, pay attention to the material and venting method.

According to the design, we can conclude three things. The first is one commonly seen in catalogues.

When calculating the size of valves, the back pressure factor has to be taken into account. Balanced bellows valves are fitted with the Kb or Kw coefficient depending on whether vapour or liquid is being handled. The classical and pilot designs assume that the value is 1.0. Without it being factored in, the efficiency rating obtained will have an optimistic bias that grows with increasing back pressure.
The flow coefficient will be reduced because the bellows contributes to the blockage of the stream.
In addition to that, the valve bonnet has to be equipped with a vent, which is stated in API 526. This vent can’t be connected with the source of pressure because that would lead to back pressure in the spring chamber, bringing all advantages of using a bellows gone.

The bellows itself is constructed from thin steel and will be subjected to wearing-and-tearing, meaning it will wear out eventually. Once it fails, the valve will function like an ordinary valve, and there will be a visible vent. If the vent is located two floors above and nobody sees it, then you will have an ordinary valve converted into a balanced one without even noticing it.

Pilot-Operated

According to the information, process pressure acts upon main valve to close it. The main valve has either a piston or diaphragm plate with the inlet pressure acting on both sides. The net force acts on the valve because of the difference in dome area and nozzle area. A small pilot senses the pressure the plant is having. At the set point, the pilot opens and dumps the dome, which reverses the net force acting on the valve.

The Anderson Greenwood 2000 series and Farris pilot designs are the most commonly referenced designs in plants, especially when the pressure relief needs are high, and a direct spring valve is too big for the structure.

What it buys you

Endures backpressure as high as 90% of the set pressure.
Provides reliable sealing with normal operating pressures of above 90% of set pressure, something no normal valve can achieve.
Achieves a large orifice capacity in a small body with lower costs for flanges and support steel.
The accuracy of setting point is the highest of the three due to the ability of the pilot to read the actual inlet pressure without being influenced by backpressure.

What it costs you

There are more components in this; they all serve the purpose of avoiding problems encountered with standard valves.
Sensing lines must be protected from clogging, freezing and corrosion; if a pilot line becomes clogged in winter, it renders the whole valve a decorative piece.
Certain designs require the pilot to have clean, dry gas; instrument air which would work in the cylinder would be insufficient in this case.

The advantage comes in at operating pressures close to the pressure set point, very high or varying backpressures, large capacity, or all of these factors combined. LNG and large ammonia service applications have persisted in this direction as the set pressures and capacities have increased with time.

Construction: Direct-Acting, Piston, Diaphragm

A non-actuating axis, which is even more significant than the actuation axis in sanitary applications. There are three technologies to meet practically every need that you will encounter.

Direct-acting

A flat diaphragm or small bellows applies directly on the valve seat; there are no external sensing lines involved. This is the simplest and cheapest of the three types of technology, and it is suitable for pressure within a certain range – from medium to low level and typically with good accuracy up to ±10% of the set pressure. This works fine in case of pump discharge or utility pressure lines; however, in practice, if a vessel operates with 5% difference in operating and set value, this solution may not work and cause losses.

Piston type

The pilot and principal valve have a singular body whereby the pilot exhaust pressure activates a piston that controls the opening of the main valve due to the inlet pressure. Even though it employs the same size of pipe, due to its unique working mechanism, it allows the higher flow rates compared to direct acting units thus making it a compromise solution between basic direct-acting device and fully piloted system. This pneumatic valve works when the flow rate required is borderline but the budget available does not support a piloted design solution.

Diaphragm type

A flexible diaphragm separates the upper works and spring from the process. It is the common choice of sanitary, pharmaceutical and corrosive applications. When people refer to a "sanitary relief valve," they typically have an Alfa Laval model in mind. There are no voids in which product could penetrate into the bonnets, and all surfaces can be made to the required degree of hygiene with appropriate treatments.

Safety Valve, Relief Valve, Safety Relief Valve

The third variable identifies the product of the device and the medium it protects. The terms can be used limply for the purposes of using these three names. The term itself is not the point.

Device	How it opens	Medium	Where you'll see it
Safety valve	Pop action, sudden and full	Compressible: gas, steam, vapour	Boiler drum, steam header, compressor discharge
Relief valve	Gradual, proportional to pressure rise	Incompressible: liquid	Hydraulic circuits, pump discharge, liquid ammonia
Safety relief valve	Either, depending on trim	Two-phase, or where the phase can change	Refinery columns, hydrocarbon storage, two-phase dumps

This is not nitpicking since the liquid needs appropriate opening, due to the absence of expansion when released; if the valve in liquid usage is pop-action, it latter creates hydraulic impact followed by chatter in the seat. On the other hand, gas or steam application requires the opening being pop, so that it gives full amount of flow after the header was released. The wrong one will successfully pass the test at the plant but misbehave during work.

The acceptable overpressure varies according to each application and defines the maximum set pressure you can implement. For example:

10% for single relief in normal code operations
16% for multiple devices including automatically controlled valves used solely for overpressure situations
21% for fire situations
3 % for steam boiler applications as per ASME Section I, which is much stricter and catches people out who have only worked under Section VIII.

In Section VIII System, a single device can allow the vessel to accumulate up to an impressive 110% of MAWP and 121% in case of fire. It is the value of the accumulated pressure that defines whether your piece of equipment can be certified. Many inquiries are received with information only on the set pressure while consideration of accumulation pressure leads to a completely different answer.

Hygienic and Sanitary Designs

Sanitary service imposes conditions that are unrelated to pressure but correlate notably with cleanability. There shouldn't be any cavities where accumulation of product can occur. The surfaces becoming wet must have a specific roughness as Ra 0.8 µm or lower as defined by 3-A practice. It is necessary to have a diaphragm separating the spring from the body. In practical terms, there must be 316L produced by tri-clamp or hygienic thread manufactured according to 3-A or EHEDG standard.

Most sanitary safety valves can be mentioned as direct acting diaphragm valves used to protect rotary positive displacement pumps, CIP systems, heat exchangers, and vessels in dairy, beverage, pharmaceutical, and biotechnology plants. They are the type that is often incorrectly specified because of one major reason – the industrial safety valve can pass the pressure test but it can fail the cleanliness inspection and the person who approved the purchase order of the equipment never knows about the purification audit performed.

The construction details and the compliance paperwork for that category are covered separately in this guide to what a sanitary pressure relief valve is and where it belongs.

Vacuum Relief

Both pressure and vacuum tank failure exist, and the latter is surprising because one cannot see a leak. The vacuum pressure valve allows air into the tank whenever internal pressure is less than atmospheric pressure to prevent it from imploding during pump outs and steam condensations.

Vacuum sizing is different from the pressure relief sizing. The maximum liquid extracting rate or condensation rate determines the vacuum rate when the API 200013 is used for atmospheric pressure tanks. The figures to be set are in millibars and only slightly above the atmospheric pressure.

Both types of failure can be found in both cases. The deeper the setting is, the more possibility for the tank shell to rupture before the valve opens. If set up too shallow, it gives continuous leak of air.Where both directions need protection a combined breather saves a nozzle and a connection, and that trade-off is set out in this explanation of combined vacuum and pressure relief valves.

Rupture Discs

A membrane made of thin metal that ruptures when it reaches a certain pressure and then remains ruptured without closing again. This isn't the same as a valve. But still, one can use it for at least 3 applications.

When the capacity needed is greater than that provided by a valve
When the process is corrosive or fouling that it is better not to use a seat in this process
When it is necessary to have a leak-tight barrier before the valve so that it does not get in contact with the process

The price to pay is that the plant is shut down while this device is in use. It works if the tolerance is ± 5\% or even tighter if you want to pay extra. This means that one must be very careful when working with a valve rated at 10 bars in a system that works under pressures of 9.2bars. The best choice is to place this device upstream a relief valve with a pressure gauge measuring the pressure in between them.

Material Follows the Fluid

Pressure rarely decides the body material. The fluid does, and so does the weather.

Material	Right for	What bites you
Brass / bronze	Water, air, steam, small sizes, non-aggressive fluids	Dezincification in high-chloride water; never hygienic
Cast or ductile iron	Large bodies, water and steam at moderate conditions	Corrosion in aggressive service; cast iron fails brittle
Carbon steel	Steam, air, oil, general industrial	Needs coating or lining for anything corrosive
304 stainless	General corrosive duty, food-adjacent	Chloride pitting; wrong choice for brine or seawater
316 / 316L	Hygienic, pharma, marine, chemical	Still pitted by chlorides under deposits and at temperature
Duplex, super-duplex	Seawater, high chloride, high strength duty	Price, and thin availability in small sizes
Hastelloy and similar	Strong acids, chlorinated organics, hot corrosive service	Price and lead time
PTFE-lined	Aggressive chemistry where metal won't survive	Temperature ceiling; mechanical damage risk

Brass needs to have a warning since it's the first choice for cheap water applications but goes bad in a way that confuses it with something else. The dezincification process dissolves the zinc from the alloy leaving behind a copper sponge entity that leaks through solid metal. The chloride exposure is a gradual process. A brass relief valve working with brackish water will probably serve for five years but looks like it can do its job for 20 years.

Stainless steel requires knowing the grade rather than the family. When choosing from the grades 304 and 316 chloride exposure is a crucial factor rather than the temperature.The performance differences and the selection logic are worked through in this guide to stainless steel pressure relief valves.

Installation Mistakes That Undo a Good Selection

You may think that you have chosen the appropriate valve, but still end up experiencing problems, since most of the issues relating to the plant's performance are in fact caused by piping.

The 3 percent rule is already out of the question. There may be long inlet runs with a reducer right at the flange, the inlet flow would also be interrupted by gate valves having a small diameter thereby making the pressure drop during operation to exceed 3 percent. This results in a situation where the valve opens late and shakes creating problems but all the blame is placed on the datasheet.

A second issue emerging here is the size of the discharge pipe. Built-up pressure is a result of the execution of your own outlet piping solution.

Furthermore, there are numerous important issues that actually seem to be rather small. First of all, you need to mount the valve with its stem vertically as if it is done horizontally, the valve would not work properly since it would exert excess stress on the guide.

How to Choose, In Order

There are a total of six questions and each one is crucial in succeeding the next, in eliminating the candidates.

What type of fluid is involved? Liquid, gas, steam, two-phase, slurry. This helps set between safety valves and relief valves.
What is the set pressure and to what extent does the facility operate above the set pressure? Once 90% of the set pressure is crossed, this means there are no more options with the conventional valve; pilot-operated or tighter seating trim must be used. This question will eliminate most of the economical options.
What about the back pressure? Less than 10% means conventional, 10-50% means bellows, above 50% means pilot-operated. This must not be assumed but checked.
What is the capacity in the worst-case scenario? Blocked outlet, fire, thermal expansion, gas blowby, runaway reaction. Each of them means different relief loads and the maximum one should be used for choosing the size.
Where does the gas go, to the atmosphere, closed header, flare, catch tank? This question affects the previous question and in hygienic services determines whether it is acceptable for the outlet to be open at all.
What requirements to the environment must be met- wash down, coastal air, outdoor installation, hygienic finish, fire rating?

The output used in the installation must be checked according to the fact that this instrument only operates one time. It is the last defense, since all gauging devices and personnel had failed by then.What a drifting valve looks like before it becomes a failure is covered in this note on how to tell whether a pressure relief valve has gone bad.

Brands and What They Cost

Three tiers, and the tier should follow the consequence of failure, and the pipe size has nothing to do with it.

Tier	What's in it	What it costs
Small brass and bronze	Compact hydraulic and pneumatic relief valves, 1/4 to 1 in, adjustable	Roughly $40 to $200 a unit
Sanitary stainless	Diaphragm type, tri-clamp, 316L, CIP-capable	$200 to $600 for a 1 in unit, more at 2 in and above
Process, code-stamped	Bellows and pilot-operated, flanged, ASME-stamped, API 526 orifices	$1,500 to $4,000 typical; five figures for a large pilot-operated assembly in an exotic alloy

For the process side of the business, prominent names that can be found in datasheets are those of Emerson's Crosby range of products, LESER, Spirax Sarco, Anderson Greenwood and Farris. For steam applications, Spirax Sarco's valve SV615 can be found in many small steam plants across Europe. On the sanitary front, there is a completely different list, where the emphasis is on cleaning rather than power; this includes names like Alfa Laval, GEA, KIESELMANN and many other producers of sanitary flow components.

Some buying takeaways that I can offer are that an ASME stamped valve refers to a special kind of product that differs from its non-stamped counterpart and that the stamp implies a significant expenditure which is relevant only in case if it is specifically required by the code. Also, within tiers, other than the price, the difference there is trim and paperwork, rather than quality, such as 316 trim, soft seat, bellows, and a capacity test which may lead to increasing the cost of the same body by twice.

FAQ

What are the three main types of pressure relief valves?

Classification based on what lies in between the spring and disc divides installations into three types: a simple spring directly acting on the disc, a spring encased in bellows, or a small pilot valve, which performs the measuring for a major valve. The real difference between them is the outlet pressure. A simple spring can withstand about 10% of the set pressure, while the bellows type can take between 10 and 50%, and the pilot type can be used up to 90%. Construction and materials are separate questions and this mixing school often becomes the cause for many misunderstandings with catalogs.

What is the 3 rule for pressure relief valves?

Typically, the 3% inlet rule according to API 520 Part II specifies that the pressure difference between the vessel being protected and the inlet valve at its rated capacity must not exceed 3% of the set action point. Pilot-operated valves tend to prefer even less pressure drop since a valve with a low inlet pressure will open later than intended, allowing the vessel to exceed limit pressures. Complicating matters, some plants use “3” to represent the three-year inspection period, so make sure to check which context applies before quoting a number at the meeting.

What are the 7 most common types of valves?

In general, relief valves belong to the controlling family that includes such devices as a gate, globe, ball, butterfly, check, plug, and diaphragm valves. Therefore, ask for a list of relief devices exclusively, as this list then says: plain spring relief valves, bellows relief valves, pilot-operated relief valves, direct-acting diaphragm relief valves, piston relief valves, vacuum relief valves and rupture relief valves.

What kind of pressure relief valve do I need?

Open whereas the gas or steam would not. Next, take a look at the discharge point. If the back pressure is low and steady, then it will keep things cheap; a normal header changes everything when it comes to bellows; anything over 50% of the set pressure will make you utilize the pilot system. If you operate within a 10% range of the setpoint then you are creating a condition that would require a tight-seating system irrespective of the header system used.

References

Conclusion

There is nothing difficult about the kinds of relief valves if you see it through the three lenses of actuation, assembly and material. However, the first thing is to sequence everything properly. Backpressure is responsible for the type of valve that would suit the application. The piping system is already built, so this is the first parameter to be put in place. The fluid and hygiene requirements are responsible for the assembly's design. Chemistry impacts the choice of materials; the aggressive nature of service determines costs.

Now, you need to size the orifice based on the governing condition instead of pipe diameters, define the amount of accumulation that the code permits according to local rules, and know about the installation requirements by the time the valve arrives to your site instead of learning about them after the valve starts cycling. Finally, the last aspect is the brand of the valve, as the certificate provided by the manufacturer.