Guidelines for Pressure Relief Design

16.   Relief System Layout-General Code Requirements-UG-135 Installation

   A.     Spring loaded safety and safety relief valves is Section VIII service should be installed in the upright position with                   the  spindle vertical. 

   B.     The valve may be installed in other than vertical position if, a) the valve design is satisfactory for such position and                is acceptable to the Manufacture of the valve., b) the media is such that solid material will not accumulate at the inlet              of the pressure relief valve, and c) drainage of the discharge side of the pressure relief valve body and discharge                      piping prevents collection of liquid on the valve disc or in the discharge piping.

  C.     Typical single safety and safety relief device installation is when the valve or disc is mounted on top of tank.

  D.     Safety, safety relief, pilot operated, rupture disc, and non-reclosing pressure relief devices are to be connected to                  the  vapor space of the protected equipment.   Liquid relief valves are to be connected below the liquid level.                              Alternative connection locations are permitted, depending on the potential vessel overpressure scenario and the                    type of relief device selected, provided the requirements of UG-125(a)(2) and UG-125(c) are met.

  E.     The “Code” requires the Inlet and outlet pipe size to and from the relief device must be the same size or larger as the               relief device inlet and outlet connections.

   F.     ASME BPV Code requires liquid relief valves are to be at least ½” pipe size.  Typical ½” valves may give more flow              than required, but smaller pipe sizes often do not withstand the rigors of field service.

  G.     The flow characteristics of the upstream system shall be such that the cumulative total of all non-recoverable inlet                  losses shall not exceed 3% of the valve set pressure.  The inlet losses will be based on the valve nameplate capacity                corrected for the characteristics of the flowing fluid.  This flow is also referred to as the relieving capacity (or rated              capacity) at 10% over-pressure.

  H.     In non-mandatory Appendix M the 3% limit on inlet loss is applied to pilot operated relief valves as well as to the                       common safety relief valves. 

   I.     The opening through all pipe, fittings and no-reclosing pressure relief device (if installed) between a pressure vessel               and its pressure relief valve shall have a least the area of the pressure relief valve inlet.

  J.     When two or more required pressure relief devices are placed on one connection, the inlet internal cross-sectional                 area of this connection shall be either sized to avoid restricting flow or made at least equal to the combined inlet                     areas of the safety devices connected to it.

K.    The pressure relief devices on all vessels shall be so installed that their proper functioning will not be hindered by the            nature of the vessels content.

L.    Discharge lines from pressure relief devices shall be designed to facilitate drainage or shall be fitted with drains to                  prevent liquid from lodging in the discharge side of the pressure relief device, and such lines shall lead to a safe place            of discharge.

M.    Use a Three-way Tansflo-valve when it is necessary to service or change relief device without taking a vessel out of             service. They are open to either one valve or the other providing opportunity to take either valve out of service for               testing, repair or replacement.

N.    A multiple relief valve rupture disc combination can be used to protect two separate scenarios using different set                    pressures.

O.    Complex relief valve arrangement can be used in conjunction with automated ball valves.

P.     Use an over-head hook arrangement to connect discharge lines to relief headers to protect against liquid flowing                   from header to relief discharge.

Q.    A stop valve may be placed in discharge line to protect headers only by special approval and must be installed in                       locked open position.

R.      Low pressure rupture disc can be placed in relief valve discharge pipe to protect against liquid back-flow from                          header lines.

S.       Non-Fragmenting rupture can be installed directly above the tank nozzle while fragmenting rupture disc should be                  installed at a 90-degree angle away from the tank.

T.      Avoid use of rupture disc requiring cutting bars. Never use if system can be liquid full.  If used in gas service, establish            procedures to check for proper installation.

U.     Do not use scored reverse-buckling disc in liquid service unless it is recommended by the manufacture.

V.     Rupture disc devices should not in general be used for liquid service unless specifically designed for such service and           the model disk is recommended by the manufacture

W.    Use a Transflo valve for cost savings to alternate between a relief valve and rupture disc when needing to take relief             valve out of service for repair or inspection.

X.     In contrast to pressure relief valves there are no restrictions on the associated piping with regard to operability of                 the device provided the flow calculations meet the calculation requirements described in the ASME BPV Code.  In the             case of restricted-area devices, installing the device in an expanded section of the piping can be advantageous.

Y.     For rupture disc only arrangement use 90% of actual flow when comparing actual flow vs. required flow rate.

Z.     For API 520 Section “Coefficient of Discharge Method” for sizing a rupture disc.  When using this method,               the coefficient of discharge is Kd = 0.62 and the rupture disc must be installed within 8 pipe diameters of the nozzle               entry and the discharge must be less than or equal to 5 pipe diameters. 

AA. For longer piping arrangements use API 520 Section  “Rupture disc sizing using Flow Resistance Method” -                 For this method the designer must take the total piping resistance and determine the maximum flow that can pass                   through that piping resistance and then reduce the flow by 10%.  This flow must be equal to or greater than the                       required relief capacity.  The designer must use the certified flow resistance of the rupture disc from the                                     manufacture, entrance and exit losses, along with pipe fitting resistances to determine the total resistance to flow.

BB. When rupture disc device are used, it is recommended that the design pressure of the vessel be sufficiently above the            intended operating pressure to provide sufficient margin between operating pressures and rupture disc bursting                      pressure to prevent premature failure of the rupture disc, due to fatigue or creep.

CC. Use a telltale assembly when installing a rupture disc beneath a safety or relief valve to detect and signal any pressure          changes.

DD. Make sure that relief systems are properly supported against peak thrust loads during relief flow and allow for                        thermal expansion in thrust restraint design.  Installation may require computation of the bending moments and                        stresses in the piping and vessel nozzle.  There are momentum effects and pressure effects at steady state flow as                  well as transient dynamic loads caused by opening.

EE.  Devices operating at or below 15 psig are outside the scope of the ASME Code.  The characteristics of typical types              of devices are covered in API Standard 2000.

FF.  For API storage tanks the designer must also take proper account of the effect of overpressure on the apparent                     coefficient of discharge if the overpressure is insufficient to fully open the valve, as much as 200% overpressure can             be required for full opening).  The set pressure must be well below the vessel design pressure to permit such                             overpressure.

GG. Typical low-pressure devices are not intended for liquid service, so liquid flow capacity data are generally lacking.  If               there is no over-flow connected to the tank, the designer must assure that the liquid flow from the high point of the             relief system will not create a static head in excess of the allowed pressure accumulation.


17.     Relief Device Maintenance Process 


A.  Install tail-piping with removable spool sections or tees with blank flanges to facilitate inspection and cleaning.
B.  The use of block valves in relief systems to facilitate maintenance is not recommended and should be avoided.  See                  ASME if exception is required.

C.  If a design is inadequate because fluids can freeze up in the system during normal operation, the addition of steam or            other tracing will not make the design acceptable. (flush mounted disc etc.).  Tracing can be added for maintenance                  purposed, but the relief must be functional if the tracing system fails.
D.  Make sure siphon breaks are present in overflow pipes on low-pressure tanks, particularly if the pipe is to provide                  vacuum relief.  Siphoning can collapse tanks.