Built-up back pressure

This is the increase in pressure at the outlet of the PRV as a result of flow after the pressure relief device or devices open. 

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Back Pressure - Is the pressure on the discharge side of a pressure relief valve. Total back pressure is the sum of superimposed and built-up back pressures. 

Built-up Back Pressure - Is the frictional pressure drop that develops as a result of flow through the discharge system after the pressure relief valve opens. 

In applying this rule, the capacity of the pressure relief system must also be sized to handle the quantity of fluid released at this pressure (together with other expected loads during this contingency), so that the built-up back pressure will not result in exceeding 1.5 times the design pressure. This additional load need not, however, be considered in calculations of flare and PR valve radiant heat levels. 

The built up back pressure exceeds 10% of the set pressure, based on psig or it exceeds 21 % of set pressure in the case of fire. 

When the pilot exhausts to the atmosphere, a pilot-operated PR valve is fully balanced. Like the balanced bellows valve, therefore, its opening pressure is unaffected by back pressure, and high built-up back pressure does not result in chattering. 

Conventional PR valves and discharge systems should be designed such that built-up back pressure does not exceed 10% of set pressure (both measured in psig), to avoid chattering problems. In the case where a pressure relief valve system is sized for fire conditions, with 21 % overpressure, built-up back pressure up to 21 % of set pressure is permissible. However, the lower rates resulting from other contingencies still must meet the 10% limitation. 

Balanced bellows PR valves need not be restricted to the same built-up back pressure limit (10% of set pressure) as are conventional valves, since they are not subject to chattering from this cause. However, maximum back pressure is limited by capacity and in some cases by the mechanical design strength limitations of parts such as the outlet flange, bellows, or valve bonnet. 

Excessive Built-up Back Pressure - Built-up back pressure resulting from discharge flow through the outlet system of a conventional PR valve results in a force on the valve disc tending to return it to the closed position. If this returning force is sufficiently large, it may cause the valve to close, only to reopen immediately when the effect of built-up back pressure is removed. Chattering results from the rapid repetition of this cycle. 

For the exceptional cases of subcritical flow (e.g., where a PR valve is designed for a low set pressure and the total superimposed plus built-up back pressure exceeds the critical flow pressure), the following equation may be used applied ... 

Built-up back pressure Pressure existing at the outlet of a pressure relief deviee eaused by flow through that partieular deviee into a diseharge system. 

Vessel is final scrubber in a flare, relief, or vent system and is designed to withstand maximum built-up back-pressure. 

Vessel is final scrubber in a flare, relief, or vent system, is designed to withstand maximum built-up back-pressure, and has no internal or external obstructions, such as mist extractors, back-pressure valves, or flame arrestors. 

Assume that the back pressure on the safety, valve will be limited to 10% of the gauge set pressure. (The back pressure required is the sum of the constant and built-up back pressure. If the relief system discharges direct to the atmosphere, then the constant back pressure will be zero.) If the relief pressure is 4.4 barg, the set pressure is 4.4/1.1 = 4.0 barg, and the maximum back pressure is 0.1 x 4.0 = 0.4 barg. Thus 4... 

The pressure of the environment downstream of a relief pipe or relief device, into which the pipe or device discharges., Back pressure can be "constant" or "built-up". A constant back pressure always exists, irrespective or the relief process, e.g. atmospheric. pressure. A built-up back pressure, exists due to the frictional pressure drop caused by flow through the relief system. 

Where conventional Safety Relief Valves are used, the relief manifold system should be sized to limit the built-up back pressure to approximately 10% of the set pressure... 

Built-up Back Pressure Units 50. Selected Orifice Area (in square in.) ... 

When the cross-section of the flow channel decreases, the flow will proceed against a built-up back-pressure. This back pressure, which reaches its maximum at some distance before the narrowest part of the gap, forces the material through the gap, while it obtains a higher speed than the circumferential roll speed. A part of the material supplied by the upper roll, flows back from the location of the pressure peak, and arrives at the bank in front of the gap, in which it circulates (see Figure 11.21). 

A bellow style pressure relief valve is required to protect a vessel containing an organic liquid. The required relieving capacity is 310 gpm. Inlet temperature is 170°F. Set pressure is 100 psig. Allowable overpressure is 25%. Built-up back pressure is 25 psig. Specific gravity is 1.45 and viscosity is 3,200 cP, Determine the orifice size of the valve. The correction factors are ... 

One unique advantage of the pilot-operated PRV is that the lift of the piston does not depend on the back pressure of the system and the performance of the valve is not affected by the built-up back pressure. 

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