PRV scenarios

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Pressure Relief Valve Scenarios

SEPTEMBER 26, 2010

Matt Buchanio currently serves as Pressure Relief Valve marketing director for

Tyco Valves and Controls since 2006. He started his career with Crosby Valve & Gage Company in 1975and has held roles in Manufacturing, Regional Sales, National Account Management and Product

Management. He can be reached at 508 384-4669 or [email protected].

Q: "Pressure relief valve" is a rather generic phrase. How would you describe pressure-relief valves in

more specific terms, and in what sort applications are safety valves typically employed?

The term pressure-relief valve is often used to describe three specific types of valves:

Safety Valve: A safety valve is a pressure relief valve actuated by

inlet static pressure. It is characterized by a rapid opening or pop action and is normally used on steam

and air applications. Safety valves are commonly used to protect the drum and superheater sections of

steam generators.

Relief Valve: A relief valve is a pressure relief device actuated by the inlet static pressure, having a
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gradual lift generally proportional to the increase in pressure over opening pressure, and is primarily used

for liquid service applications, such as overpressure protection of a positive-displacement pump, for

example.

Safety-Relief Valve: A safety relief valve is a pressure relief valve characterized by rapid opening or pop

action, or by opening in proportion to the increase in pressure over the opening pressure, depending on

its application and may be used either for liquid or compressible fluid service.

Q: How has pressure relief valve technology evolved over the past 10-20 years? How are the

pressure-relief valves of today more effective/efficient in terms of their ability to ensure process safety

than previous generations of pressure-relief valves?

Over the past 20 years, there has been a significant rise in the specification and application of soft seat

pilot operated pressure relief valve technology. This trend is a result of the many advantages pilot

operated pressure relief valve designs provide when compared to traditional direct spring operated

pressure relief valves. Several of these advantages include:

System pressures can be operated nearer set pressure without valve leakage. Less expensiveinstallation costs resulting from reduced weight and lower overall height.

Modulating-action pilot operated pressure relief valves are designed to relieve proportionallyaccording to relief demand and minimize emissions.

Availability of full-bore orifices. Reduced cost of ownership since soft seats and seals extend service life and eliminate time

consuming refurbishment of metal seats.

Q: What are some best practices you can offer end-users in the areas of specification, installation and

maintenance of pressure-relief valves to ensure long-term performance?

With regard to pressure-relief valve maintenance, it is necessary to establish an effective program for

periodic inspection and maintenance for these critical devices to ensure their operational readiness.

Within the industry, there are a number of guidelines that provide recommendations for pressure relief

valve inspection and maintenance programs. TheNational Board of Boiler and Pressure Vessel

InspectorsVR standard includes an accreditation and certification process. The application of the VR

stamp to a repaired pressure relief valve provides the user with evidence of compliance to the program.
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In some cases, owner/users have developed their

own requirements and guidelines for repairing pressure relief valves based on their operating experience,

safety policies and detailed maintenance & repair records.

Finally, the owner/user should always insist on the use of new and unused, original equipment

manufacturers spare parts. The sale of refurbished, re-manufactured or reverse-engineered valve parts

by companies not authorized to do so by the manufacturer continues to be a concern in the valve industry.

The leading pressure relief valve manufacturers generally do not authorize any company to manufacture

replacement spare parts for their valve products. Use of OEM spare parts ensures interchangeability with

the original part, the correct materials of construction, and proper heat treatment procedures, etc.

Q: What are some pitfalls that you see end users commonly encountering in pressure relief valve

applications? How can end users best avoid and/or respond to such application pitfalls?

One of the more common pitfalls with pressure relief valves pertains to installation orientation on valves

required to meet ASME Code Section VIII requirements. Manufacturers are routinely contacted by

customers questioning whether pressure-relief valves can be mounted horizontally. Unfortunately, these

inquiries usually occur after the valve has been installed on the protected vessel or system and possibly

rejected by the local jurisdiction. The horizontal orientation is usually the result of inexperience on the

part of the company performing the installation, clearance constraints, or the relief system piping

configuration precludes the valve from being installed in the vertical position.


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Most pressure relief valves are not designed for horizontal

mounting and there are two concerns with this type of orientation:

1. The valve may not have the ability to properly drain after a relief event.

2. The performance of the valve specifically seat tightness integrity can be adversely affected when

mounted in the horizontal position. This is especially true for metal-seated valves, where precise

alignment of the valve components is necessary to meet industry-recognized seat tightness requirements.

Our experience with valves mounted horizontally is that after an initial overpressure event, the valve may

not reseat properly, thus causing the valve to leak.

Most pressure relief valves are designed, tested and capacity-certified in the upright position with the

spindle vertical and should be installed accordingly.

Q: How, if at all, does the sizing and selection of pressure relief valves differ from the sizing and selection

of more traditional valve styles? What are some key considerations the end-user should make when

sizing and selecting pressure-relief valves?

Due to the variety of service conditions, there is no one type of pressure relief valve that can be applied in

every installation. In general, most overpressure protection applications can be handled with either a

direct spring-operated or a pilot operated pressure relief valve design. There are numerous factors that

are taken into consideration when selecting a pressure-relief valve:

Operating Pressure/Set Pressure Differential Pressure relief valve blowdown characteristics Temperature Inlet pressure drop Seat tightness requirements Viscosity Multiple relief scenarios/contingencies Backpressure conditions


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Seat configuration requirements (metal versus soft) Vibration Multiple valves/cost of inlet and outlet piping manifolds Material compatibility with the lading fluid Hazardous or lethal service Presence of foreign particles or solids Polymerization Installation clearance constraints

Pressure relief valves must be designed and selected with construction materials compatible with many

process fluids ranging from simple air and water to the most corrosive fluid. They must also be designed

to operate in a consistently smooth and stable manner on a variety of fluids and fluid phases. Data

compiled by the manufacturers of each type of valve under consideration should be consulted for

information relative to its performance under the conditions anticipated.

Q: What is on the horizon in terms of pressure relief valve technology? How will the pressure relief valves

of tomorrow be more effective/efficient in ensuring overall process safety than the pressure relief valves

of today?

The trend of higher usage of pressure relief valve designs with resilient seats and seals is expected to

continue as a result of more stringent emissions requirements.

We also expect the use of pilot operated pressure relief valve technology to expand into applications

traditionally handled by spring operated designs, notably power applications. To some degree this has

already started in certain power plant applications. Finally, the growing demand to produce more and

more electricity has resulted in new generation power plant designs requiring new steam generator

designs with elevated design temperatures, requiring high temperature alloys. Future safety valve

designs will have to be developed to suit these applications.

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