Installation Practices for Safety Valves

Safety Valve Installation Applications A safety valve is the primary component of an overpressure protection system. Regardless of safety valve design, the inlet and outlet piping can determine the level of protection provided and the quality of the safety valve's performance.

Inlet Piping

Correct design of inlet piping in safety valves is extremely important. Often, safety valves are mounted at the location most physically convenient to the installation, with little consideration given to the effects of inlet pressure drop on the valve's performance. Pressure drop always occurs in the flow in the pipe connected to the safety valve inlet. Depending on the pipe diameter, geometry, and internal surface condition (roughness), the pressure drop can be very large (more than 10%) or very small (less than 3%). API 520 Part II and the non-mandatory section of ASME Section VIII recommend a maximum inlet pressure drop of 3%. However, it is extremely important to note that the mandatory section of ASME Section VIII does not set a limit on inlet pressure drop as long as the PRV discharges the required capacity and operates stably!

Outlet Piping

Outlet piping is at least as important as inlet piping. Most Anderson Greenwood Crosby safety valves perform better than other commercial valves even when piping conditions fall below standards. Although some valves appear to use very little discharge piping - such as an elbow or short, vertical riser - significant backpressure can develop. Nearly all conventional safety valves will reduce discharge capacity or close prematurely at backpressure as low as 10% to 15%. This causes irregular valve operation and chatter. For this reason, API and ASME recommend limiting backpressure in discharge piping to 10% for conventional unbalanced safety valves. Although Anderson Greenwood Type 81/83/86 valve designs can tolerate somewhat more accumulated backpressure than other designs, this rule should be kept in mind. Additionally, raising the safety valve's blowdown setting can compensate for some of the accumulated backpressure.

Valve Installation Applications

There are several important considerations regarding the physical installation of safety valves that will help ensure their proper performance. Let us examine some of the most important ones.

Safety Valves Must Be Mounted Vertically and Facing Upward

While this may seem straightforward, it is worth mentioning because incorrect installations are encountered. Unless the manufacturer approves an alternative arrangement, all PRVs must be mounted in a vertical plane with the inlet facing downward. Pilot-operated safety valves should also always be installed with the pilot vertical and facing upward. This is shown in all PRV drawings in Anderson Greenwood Crosby literature. Installing a valve horizontally or upside down is incorrect practice and directly contradicts recommendations from API, ASME, and Anderson Greenwood Crosby.

Safety Valves Must Be Properly Supported

All safety valves must be installed to minimize bending and stress. Particularly in direct-acting spring-loaded safety valves, bending stress can increase resistance (friction) between moving parts by stretching internal components and/or the body. The set pressure can increase; wear during discharge can increase; or stress can completely prevent the valve from closing. Bending stress can arise from three causes. 1. Physical Load The safety valve may be supporting connected piping. To prevent this, inlet and outlet piping should be independently supported. 2. Thermal Expansion Most safety valves are at ambient temperature until they are activated. When activated, fluid flow will heat or cool the valve and adjacent piping. This will cause the valve and auxiliary piping to expand or contract. If structural support prevents some movement of the piping, high stresses can be imposed on the safety valve. 3. Reactive Forces These forces can develop when the safety valve is activated. Similar to thrust from a rocket or jet engine, these reactions occur from the flow of fluid through the valve and changes in flow direction. Thrust on the valve and connected piping will create bending stress. Proper and strong placement of supports can prevent these.

Storage and Transportation

Until a safety valve is installed, it should be kept in dry storage away from contamination. This will prevent rust and the entry of particulate matter such as sand and insects. Anderson Greenwood Crosby closes all openings as standard practice when necessary and provides flange protection. Special packaging is provided for demanding environments such as maritime transport. Safety valves are sensitive products that exhibit performance loss if misused. Safety valves should not be dropped or subjected to impact, as this can damage the body and cause moving parts to lose their properties. Pilot-operated safety valves typically are used with external piping and various accessories. During transportation, careful attention must be paid to these additional components. Direct-acting safety valves with metal seats must also be transported vertically and stored in an upright position. The vertical position is necessary to minimize the likelihood of damage to seating surfaces. Further information can be obtained from Anderson Greenwood technical seminars. You can find other application recommendations and technical information in the product datasheets or from us. Standards and References Regulatory Body Codes and Standards Regulatory Body Codes and Standards Regulatory Body – Codes and Standards • American National Standards Institute (1430 Broadway New York, NY 10018) - B16.34 Steel Valves, Flanged and Buttweld Ends - B16.5 Steel Pipe Flanges and Flanged Fittings - B31.1 Power Piping - B31.3 Chemical Plant and Petroleum Refinery Piping - B31.4 Liquid Petroleum Transportation Piping System - B95.1 Terminology for Pressure Relief Devices - ANSI/ASME PTC 25.3 Performance Test Code, Safety and Relief Valves • American Petroleum Institute (2101 L Street Northwest Washington, DC 20037) - API RP 510 Pressure Vessel Inspection Code - API RP 520 Recommended Practice for the Design and - Installation of Pressure Relieving Systems in Refineries: - Part I - Design; Part II - Installation - API RP 521 Guide for Pressure Relief and Depressuring Systems - API Standard 526 Flanged Steel Safety Relief Valves - API Standard 527 Commercial Seat Tightness of - Safety Relief Valves with Metal to Metal Seats - API Standard 2000 Venting Atmospheric and Low Pressure Storage Tanks - API Guide for Inspection of Refinery Equipment - Chapter XVI - Pressure Relieving Devices • The American Society of Mechanical Engineers United Engineering Center (345 East 47th Street New York, NY 10017) - Boiler and Pressure Vessel Code - Section I - Power Boilers - Section II - Materials - Section IV - Heating Boilers - Section VII - Care of Power Boilers - Section VIII - Pressure Vessels - Section IX - Welding and Brazing Qualifications • International Organisation for Standardisation (Case Postale 56 CH-1211 Geneve 20, Switzerland) - ISO-9000 Quality System - ISO-4126 Safety Valves - General Requirements • National Board of Boiler and Pressure Vessel Inspectors (1055 Crupper Avenue Columbus, OH 43229) - NB-25 National Board Inspectors Code - NB-65 National Board Authorization to Repair ASME and National Board Stamped Safety Valves and Relief Valves • National Association of Corrosion Engineers (P.O. Box 1499 Houston, TX 77001) - NACEMR0175 Mehmet Berk Güven Field Sales Engineer Pressure Management Emerson Automation Solutions