Valve inlet

The redueed inlet valve flow, q ean also be ealeulated by eompensating the redueed expander flow, q exp Rom expander inlet eonditions to valve inlet eonditions in other words, from eonditions at point 1 to the eonditions at point 0. It is already assumed that Tq = Tj and Z = Zj and, therefore, it is only neeessary to eompensate for pressure from Pj to P. This ean be formulated as ... 

The eriterion of equal mass flows before and after the breaker opening is the same as keeping the total redueed flow through the inlet and bypass valves equal. This is only true when valve inlet eonditions are the same, whieh they are in this ease. 

Figure 1. Enter on the abscissa at the water vapor pressure at the valve inlet. Proceed vertically to intersect the curve. Move horizontally to the left to read rc on the ordinate (Reference 1).

G = Gas specific gravity = mol. wt./29 Pi = Valve inlet pressure, psia AP = Pressure drop across valve, psi Q = Gas flow rate, SCFH Qs = Steam or vapor flow rate, Ib/hr T = Absolute temperature of gas at inlet, °R T5I1 = Degrees of superheat, °F... 

Keep valve inlet velocity below 300ft/sec for 2" and smaller, and 200ft/ sec for larger sizes. 

Rapid eycling can occur when the pressure at the valve inlet decreases at the start of relief valve flow beeause of excessive pressure loss in the piping to the valve. Under these conditions, the valve will cycle at a rapid rate which is referred to as chattering. The valve responds to the pressure at its inlet. If the pressure decreases during flow... 

Flow Meter Orifice Plate - A flow meter orifice plate is permissible in normal process flow pressure reheving path, provided that it can pass the required emergency flow without exceeding pressure limits of the upstream equipment. However, it is not acceptable in PR valve inlets and flare headers. 

A check valve is not, however, permissible in PR valve inlet or outlet piping, or in any flare or safety valve header. 

When the predetermined system blowdown pressure is reached, the pilot valve closes, full system pressure is restored to the dome above the piston, and the piston is quickly moved to the closed position. The pilot valve pressure sensing point may be located in the main valve inlet neck, or on the shell of the vessel being protected. The valve is less affected by inlet piping pressure drop in the latter case, as described below. 

Process systems handling polymers and resins (e.g., butyl rubber or ethylene-propylene diene monomer rubbers) are often subject to plugging at dead-end locations such as PR valve inlets. In extreme cases, complete blockage of inlet piping and valve nozzle can result. This problem can be eliminated by the application of a flush-seated PR valve, in which dead-end areas are eliminated by placing the valve disc flush with the vessel wall, in the flow pattern of the contents. 

This section describes the requirements for the design and installation of pressure relief valve inlet and outlet piping manifolds and valving, including safety valve and flare headers. 

The entire inlet line must be at least the size of the PR valve inlet. When multiple safety valves are manifolded on the inlet side, the cross-sectional area of the maitifold piping must be equal to or greater than the sum of all the inlet areas of valves open to the manifold. 

Heat tracing of PR valve inlet piping should be provided where plugging by icing, deposition of wax or congealing of viscous liquids may occur at ambient temperatures. 

PR valves handling materials which are liquid or partially liquid at the valve inlet. An exception to this is made for certain thermal expansion relief valves as described below. 

The fluid handled must be all vapor at the valve inlet. 

X = Latent heat of vaporization of the liquid in the vessel, Btu/lb, evaluated at the pressure existing at the PR valve inlet under full discharge conditions. 

Pilot-operated relief valves use the pressure in the vessel rather than a spring to seal the valve and a pilot to activate the mechanism. Figure 13-6 is a schematic of a typical pilot-operated valve. A piece of tubing communicates pressure between the relief valve inlet and pilot. When this pressure is below the set pressure of the pilot, the pilot valve is in the position... 

Each relief valve should be equipped with inlet piping no smaller than the valve inlet flange size, and inlet piping should be as short as practi-... 

For these special situations properly designed rupture disks using corrosion-resistant materials can be installed both before the valve inlet as well as on the valve discharge. For these cases, refer to both the valve manufacturer and the rupture disk manufacturers. See later discussion of code requirements for this condition. 

The contents of the vessel are clean fluids, free from gumming or clogging matter, so that accumulation in the space between the valve inlet and the rupture disk (or in any other outlet drat may be provided) will not clog the outlet. 

Inlet piping is held to a minimum, with the safety device preferably mounted directly on the equipment and with the total system pressure drop loss to pres.sure relief valve inlet not exceeding 3% of the set pressure in psig, of maximum relief flowing conditions [10]. To conform to code (see ASME code. Sect. Vlll, Div. l-UG-127 [1]) avoid high inlet pressure drop and possible valve chatter ... 

Nominal pipe diameter no less than valve Inlet size... 

Inlet piping sized so that pressure drop from vessel to pressure relief valve inlet flange does not exceed 3 percent of valve set pressure... 

Typical installation avoiding process laterals connected to pressure relief valve inlet piping... 

Typical installation avoiding excessive turbulence at pressure relief valve inlet. 

If the temperature at the valve inlet is expected to be 50°F, then the temperature correction factor... 

Refrigerant pressures and temperatures at expansion valve inlet, evaporator outlet, and compressor suction and discharge... 

With no system pressure, the pilot inlet seat is open and outlet seat is closed. As pressure is admitted to the main valve inlet, it enters the pilot through a filter screen and is transmitted through passages in the feedback piston, past the inlet seat, into the main valve dome to close the main valve piston. 

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