Types, Codes & Testing

Safety relief valves protecting life and property

By Lester Millard, Hitma Group

Generally speaking, safety relief valves have been around since the 1600s in more or less the same design concept. In its primary function, the pressure safety relief valve serves to protect life and property. Acting as a 'last resort', this fully mechanical valve is designed to open based on an over pressure situation within a process pressure system, thus not only protecting life but safeguarding the investment and plant itself.
This article reviews the principles of pressure safety relief valves for spring loaded and pilot operated designs. It will cover the applicable European and American codes and standards as well as end user procedures that are key elements in establishing safety and safe selection. Testing (set pressure verification) and maintenance - important criteria once the safety valve has been installed and commissioned - will also be addressed.
Pressure safety relief valves should be taken very seriously. Manufactured from castings they may not look very sophisticated, but in their design, accuracy and function they resemble a delicate instrument whilst performing an essential role. Self-contained and self-operating devices, they respond to system conditions and prevent catastrophic failure when other instruments and control systems fail to adequately control process limits.

A brief history

It is usually supposed that the Frenchman Papin was the inventor of the safety valve, which he first applied about 1682 to his digester
The safety valve was kept closed by means of a lever and movable weight; sliding the weight along the lever enabled Papin to keep the valve in place and regulate the steam-pressure. It appears now that Papin was only the inventor of the improvements just mentioned and that safety valves were already being used some fifty years before by the German Glauber (who contributed many valuable additions to the mechanical department of chemistry). In his practise on philosophical furnaces, translated into English in 1651, he describes the modes by which he prevents retorts and stills from bursting from an excessive pressure. A conical valve was fitted, being ground air-tight to its seat, and loaded with a "cap of lead", so that when the vapour became too "high" it slightly raised the valve and a portion escaped; the valve then closed again on itself, "being pressed down by the loaded cap and so kept close".
The idea was followed up by others and we find in the art of distillation, by John French, published soon afterward in London, the following concerning the action of such safety-valves: "Upon the top of a stubble (valve) there may be fastened some lead, that if the spirit be too strong, it will only heave up the stubble and let it fall down". It should be realised that the word steam was unknown at the time, being of later coinage. In its place we find in every old book the words vapour, spirit, smoke, Ayr and even ghost, whence the modern word gas, for aeriform bodies, is doubtless derived
In the New England region of the USA, there were 1700 boiler explosions resulting in 1300 deaths during the five years between 1905 and 1911.
The American Society of Mechanical Engineers was asked to formulate a design code. The boiler & pressure vessel committee was formed and hence the A.S.M.E. Section 1 for fired vessels was formulated and became a mandatory requirement for all States, which recognised the need for regulation. The sole purpose of a pressure-relieving device (safety relief valve) is to protect LIFE and PROPERTY.
With the expansion of process industries the need for a code for unfired pressure vessels was identified which gave rise to A.S.M.E. Section VIII.
The ASME Codes are mandatory in the USA and Canada. API recommended practices and standards have been an important guidance for users and engineering companies. In many European countries, national ruIes for the protection against overpressure of process equipment were developed and remained in force well into the 20th century.
However, in order to allow free circulation of goods in the European Community, Member States were prohibited from making new technical rules and from updating the existing ones. They have to conform to a new directive, the pressure equipment directive PED which was published in 1997. lt has become compulsory for equipment "put in the market" after 29/05/2002 (refer to Art.20 - par 3 of the PED).
Today, the term safety valve should be used to describe - Safety, Safety/Relief and Relief Valves; this term is now used in European Norms (EN) and ISO 4126 descriptions. Safety valves are included as "Safety Accessories in the PED (Art. 1 par 2.1.3) and are classified in risk category IV (the highest). The manufacturer, in order to EC mark his product must undergo, for each product, a conformity assessment comprising the EC type or design examination and the assurance of the production quality system. The procedures to certify the conformity to the PED are carried out by a body notified by the Member States of the European Community.
With the completion of the above, the manufacturer may stamp the EC mark on his product.

Harmonised standards for safety valves

No standard is yet harmonised. A list of standards relative to safety valves (and rupture discs), which are in the process of being harmonised is given in Table 1. It is not compulsory to follow harmonised standards, but if a manufacturer conforms to these standards, he is presumed to conform to the PED (Art 5 of PED Presumption of conformity). Otherwise the manufacturer himself must prove that his products conform to the PED.
As a consequence of the "New Approach" there have been many changes regarding safety valves, the more significant of which being:

Discharge coefficient in back pressure conditions.

When the back pressure exceeds 25% of the full lift pressure, the manufacturer must obtain valve discharge coefficients after tests carried out in actual backpressure conditions. Reference: par 7.1.2, 7.3.3.4 and 9.1 of prEN ISO 4126-1 (see Table 1).

Table 1: CENTC 69/WG 10 Safety devices against excessive pressures

PrEN ISO 4126-1 Safety devices for the protection against excessive pressure - Part 1 : Safety valves
PrEN ISO 4126-2 Safety devices for the protection against excessive pressure - Part 2: Bursting disc safety devices
PrEN ISO 4126-3 Safety devices for the protection against excessive pressure - Part 3: Safety valves and bursting disc safety devices in combination
PrEN ISO 4126-4 Safety devices for the protection against excessive pressure - Part 4: Pilot operated safety valves
PrEN ISO D 4126-5 Safety devices for the protection against excessive pressure - Part 5: Controlled safety pressure relief systems (CSPRS)
PrEN ISO 4126-6 Safety devices for the protection against excessive pressure - Part 6: Application, selection and installation of bursting disc safety devices
PrEN ISO 4126-7 Safety devices for the protection against excessive pressure - Part 7: Common data

Bellows

If the bellows break, the valve must still open within 10% of the maximum allowable pressure of the equipment being protected. Reference: par 5.1.8 of prEN ISO 4126-1. If the valve is set at or close to the design pressure of the protected vessel (as in the case of 99% of the applications) a way to meet with requirements is to install a balancing piston to back up the bellows.

Pilot operated safety valves

It will be possible to install pilot operated valves in some Member States (i.e. the Netherlands and Germany) where local authorities have not approved pilot operated valves up to now. Reference: prEN ISO 4126-4.

Overpressure

The overpressure must not exceed 10% of the maximum allowable pressure. Overpressures such as 16% (multiple pressure relief devices) or 21 % (fire), which are acceptable in accordance with ASME VIII division 1, are not permitted.

Commonly-used terminology

  • Safety valve - steam applications, characterised by rapid, full opening or "pop" action.
  • Relief valve - liquid applications, the valves open in proportion to the increase of system pressure over the opening pressure.
  • Safety/relief valve - pressure relieving device suitable for use as a safety valve or a relief valve depending on its application.
  • Pilot-operated safety valve - in one such valve the spring provides +/-75% of the disk loading; the gas or vapour supplies the remainder through the pilot valve. When the pressure in the vessel reaches the set pressure, the pilot valve relieves the gas pressure (which contributes to the disk loading) to the atmosphere causing the safety valve to open wide. Both the pilot and the main valve contain flexible membranes and, consequently, are limited to the design factors of the membranes. These can be snap acting or modulating and are non-flowing.
  • Conventional safety valve - a conventional safety relief valve is a pressure relief valve characterised by rapid opening or pop action, or by opening in proportion to the increase in pressure over the opening, depending on the application. Such valves may be used either for liquid or compressible fluids.
  • Balanced safety valve - A balanced safety relief valve is a pressure relief valve, which incorporates a means of minimising the effect of backpressure on the operational characteristics (opening pressure, closing pressure, and relieving capacity).
  • Full nozzle - inlet flow passage; only the nozzle and disc insert are in contact with the process media when valve is in the closed position.
  • Semi nozzle - the nozzle, disc insert and part of the valve body are in contact with the process media.
  • Effective discharge area - the nominal orifice size listed in API-526, usually defined by a letter (D thru T)
  • Actual discharge area - the measured minimum net area, which determines the flow through a valve.
  • Coefficient of discharge - ratio of the measured relieving capacity to the theoretical relieving capacity.
  • Simmer - audible or visual escape of fluid between the seat and disc. Applies to valves on compressible fluids, at around 98% of the set pressure.
  • Huddling chamber - annular pressure chamber located beyond the valve seat, this generates the pop characteristics.

Safety valve nomenclature

  • M.A.W.P. (Maximum Allowable Working Pressure) - maximum gauge pressure permissible at the top of a completed vessel for a designated temperature.
  • Operating pressure - working pressure in a pipe or vessel.
    ·Set pressure - pressure at which a relieving device opens and relieves.
  • Operating gap - difference between set pressure of the valve and the operating pressure of the vessel or system.
  • Overpressure - increase over set pressure of a relief device.
  • Accumulation - increase over M.A.W.P.
  • Blowdown - difference between set pressure and re-seating pressure of a safety valve.
  • Back pressure - pressure existing at the outlet of the pressure relieving device.
  • Back pressure is either - constant or variable.
  • Built-up pressure - the pressure existing at the outlet of a pressure relief device caused by the flow through that particular device into a discharge system.
  • Superimposed - the static pressure existing at the outlet of a pressure relief device at the time the device is required to operate. It is the result of pressure in the discharge system from other sources.

Back pressure limits (variable)

Conventional 10% Built-up
Bellows 25% Built-up /Superimposed
Pilots 70% Built-up /Superimposed

Codes and standards

Because of their critical safety function, pressure relief valve design rules are very strict. Public safety laws in many countries require special inspection and verification of compliance with codes before allowing operation of the installed equipment.
The most widely used and recognised of these codes is the ASME Boiler and Pressure Vessel Code. One of the key features of the ASME Code is the rule for overpressure protection. These rules provide for the accreditation of manufacturers and the certification of pressure relief valves by tests in approved laboratories. Thus, the specification of ASME Code symbol stamped pressure relief valves assures the end-user that the performance requirements defined in the Code has been verified.
ISO 4126 provides for design and performance standards, but no accreditation process is in place to ensure compliance. Other standards groups such as NEN (Europe) provide alliterative specification and control rules. The balance of this article, however, focuses on the rules of the ASME Code as it relates to pressure relief valves. Also reviewed are contents of certain American Petroleum Institute (API) Standards and Recommended Practices, which are commonly applied in the petroleum and petrochemical industry.
For safety valves, then, the general design codes are the American Society of Mechanical Engineers (ASME) and the American Petroleum Institute (API)

The ASME code is split into two main sections:

  • ASME Section I (V) which covers Fired Pressure Vessels (Power Boilers)
  • ASME Section VIII (UV) which covers Process Installations (Unfired Pressure Vessels)

Relevant American Petroleum Institute (API) recommended practices are:

  • API RP520 Part I. This design manual is widely used for sizing & selection of relief valves.
  • API RP520 Part II. This includes methods of installation.
  • API RP521. Guide for pressure relief and de-pressurising systems.
  • API 526. Flanged steel safety/relief valves for use in the petroleum industry. Gives industry standards for dimensions, pressure-temperature rating, and maximum set pressure and body materials.
  • API RP527. Commercial seat tightness of safety/relief valves with metal to metal & soft seats.

ASME Code requirements

The important sections of the ASME Code which deal with pressure relief valves are Section I {Power Boilers}; Section 11 {Materials}; Section 111 {Nuclear}; Section IV {Heating Boilers}; Section VIII {Unfired Pressure Vessels}; and Section IX {Welding}.
As the applications of interest to the petroleum and chemical industries are primarily covered in ASME Code Sections I and VIII, only the requirements of these sections are reviewed. AII manufacturers certified to these sections of the code also comply with materials and welding requirements as they may apply to pressure relief valves. Because of the specialised nature of power boilers, the operating characteristics of pressure relief valves are very strict. Spring operated valves are generally required under Section I of the ASME Code.

Design principles

Pressure relief valves are designed to open automatically at a pre-determined set pressure level of system pressure and to achieve a rated relieving capacity at a specified pressure and temperature above the set point (overpressure) before re-closing at a pressure below the opening point (blowdown). Many manufacturers provide special trim options for liquid service valves because of the properties of incompressible fluids.
The simplest and most reliable type of pressure relief valve even to this day some four hundred years on from the first design is the spring-loaded design (Fig. 1) where a spring force opposes the system pressure acting on the valve disc. When the system pressure rises above the level of the spring force, the valve opens. This valve type may also be fitted with a bellows, for better emission control performance (Fig 2).

Spring-loaded pressure relief valve
Fig. 1 Spring-loaded pressure relief valve.

Spring-loaded relief valve fitted with a bellows and a balanced piston
Fig. 2 Spring-loaded relief valve fitted with a bellows and a balanced piston.

The significant elements of all spring-loaded pressure relief designs are the springs and the seats.
The springs must provide the desired compression rate and a reasonable range of adjustment. They must also fit into the valve bonnet and stay within the design perimeters.
The seats may be flat or angled, metal or soft. As the seat area usually defines the load transmitted to and from the spring, very high precision is essential to ensure proper valve operation.

A second type of valve, which is more sophisticated and offers operating advantages in selected applications is the pilot operated pressure relief valve (Fig. 3)

Pilot operated pressure relief valve
Fig. 3 Pilot operated pressure relief valve

This type of valve consists of a main valve and a pilot valve. The pilot responds directly to system pressure and communicates with the main valve. As with the spring loaded valve, many unique models exist. However, some common design features of pilot operated pressure relief valves include: the sensing line, the pilot valve and the main valve.
The sensing line is either connected to the valve inlet or a remote location and conveys system pressure to the pilot.
The pilot valves senses and responds to the system pressure. The pilot is the controlling member of the valve system and determines all of the operating characteristics of the valve. It consists of many small parts and passages and usually relies on elastomer seals for operation.
The main valve operates in response to the pilot and provides the main rated flow capacity to reduce excess pressure.

Application codes

The American Petroleum Institute has developed the most commonly applied standards and recommended practices for the petroleum and chemical industries in addition to the ASME Code. API Recommended Practice 521 provides excellent guidance for evaluating causes of overpressure and pressure relief systems.
API Recommended Practice 520 Part I is the design manual which is most widely used for the design. sizing and selection of components for pressure relief systems. Part II includes guidelines for recommended piping practices and methods for determining the reactive force created during valve discharge. If not properly evaluated, these reactive forces can cause chattering when conventional piping designs are applied. These high performance liquid service designs ensure smooth, stable operation and full relieving capacity on liquid service.
API also provides handling and storage recommendations. API Recommended Practice 526 provides an industry standard to manufacturers of flanged pressure relief valves and includes a common set of installation dimensions, pressure and temperature ratings, set pressure limits, capacities, and materials. This set of industry standards ensures that valves from different manufacturers will be interchangeable functionally and dimensionally. As many valves may not comply with this standard, these variables should be verified before substituting one model for another.
API Recommended Practice 527 provides a basis for testing and acceptance for set pressure and seats tightness of pressure relief valves.

Inspection and maintenance codes

As pressure relief valves contain neither instrumentation nor external operators it is necessary to establish an effective program for inspection and maintenance to ensure that they will operate when called upon in emergency situations.
A number of guidelines exist for recommending the basic structure of an effective pressure relief valve inspection and maintenance program. API Recommended Practice 510 is the Pressure Vessel Inspection Code. Also, the API Guide for Inspection of Refinery Equipment, Chapter XVI, provides excellent guidance for review of relief valves to ensure operational readiness.
The best policy for assurance of safety is also the best policy for economic considerations. The risk associated with the lack of attention to suspect pressure relief devices cannot be tolerated and is a major threat to both life and plant availability.
When purchasing new valves, always make certain that you are dealing with a reputable and reliable distributor/manufacturer. Verify that your vendor has current authorisation to sell advise/manufacture or assemble the products that you purchase.

Testing (set pressure verification)

Safety relief valve testing is one of the most important elements of an effective maintenance program. There are many techniques available for conducting pressure relief valve tests. Obviously, the most desirable type of test is one that subjects the pressure relief valves to the full operating conditions that it is expected to endure. Such a test has the advantage of assuring that all of the operating characteristics of the valve, set pressure, lift and blowdown are acceptable. However, this type of test is often impractical if not impossible.
The most practical valve test is usually the bench-testing alternative because a controlled environment can be created. By transporting valves to a central shop, consistency of test techniques and record keeping of test data can be monitored. Reference to the manufacturer's instructions for testing and adjustment is an essential part of any testing program. These documents are most easily accessed in a shop environment.
There are many contract services that provide pressure relief valve testing capabilities. It is important to review and qualify these service contractors in the same manner that valve suppliers are reviewed. The best choice is the manufacturer directly. If this option is impractical then the manufacturer's authorised service centre would be recommended. Many manufacturers offer training programs to qualify plant maintenance personnel for testing and repair of pressure relief valves.
Note that operator techniques on test benches can influence results! Therefore proper training and qualification of operators is a must for consistent and reliable test results.

Repair

In the event that a valve is in need of repair, only personnel who have been trained in the appropriate repair techniques should conduct repairs. In addition, a full reference manual of manufacturer's instructions should be accessible.
Usually, the manufacturer's instruction manual will provide limits for inspection and rework of this critical part. The seat step, seat diameters and disc holder bearing surfaces are of particular interest.
Re-machining of control rings is not recommended. If damaged, they should be replaced.
The points listed above are also applicable to pilot operated valves. In addition, all elastomers should be replaced as part of any disassembly.
Records should be kept of the "as found" adjustment of adjusting bolt and adjusting rings. This allows for resetting of adjustable members after overhaul prior to final testing.
Valves should be reassembled in the reverse order of disassembly. After reassembly of restored components, final testing and adjustment must be completed. Depending upon the test apparatus employed, final ring adjustments may not be made until after the set pressure has been determined. lf a low volume test stand is used, a "bench setting" will be employed for pressure adjustment. The rings may then be replaced in the "as found" position. AII final adjustments must be sealed. Seals should be traceable and assigned only to qualified personnel.

Records and data registration

Records should be kept which provide equipment history. This is helpful for establishing a proper inspection interval as well as evaluating the suitability of the equipment employed for the application.
Valves requiring major repair should have their inspection interval shortened. Those in excellent condition may have their interval lengthened until the optimum balance of economic and safety concerns has been established. These records are a basis for compliance with all safety concerns.

Summary

The above will hopefully provide a basic insight to the safety relief valve, its origins, design and functionality, and should hence help in the protection of life and property. Such an insight is of the utmost importance when selection and purchasing of safety relief valves for your particular plant and application.

About the author
Lester MillardLester Millard is Divisional Manager Process Control Group, Hitma Group, The Netherlands.
Following a five-year apprenticeship in Mechanical Engineering Services, he completed his training at the Colchester Institute, England.
He started work as a young engineer focusing on high-pressure water and steam systems. Later in his carrier he moved on to piping and instrumentation design, working for the major engineering contractors in the Netherlands, before specialising in procurement and later in sales of safety devices and instruments. For the past seventeen years has been working at the Hitma Group as the Divisional Manager specialising in supplying customer's valves and safety devices for the petrochemical and chemical industries. A complete glossary and additional references are available from the author, who can be reached on lmillard@hitmagroep.nl
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