Valves back pressure

P r e s s u r e g a u g e Back pressure relief valve Back pressure relief valve Digital balance... 

Since Mak s Isothermal flow chart is intended for relief manifold design, it supports calculations starting with P2, the outlet pressure, that is atmospheric at the flare tip, and back-calculates each lateral s inlet pressure. Pi. These inlet pressures are the individual relief valves back pressures. The chart parameter is M2, the Mach number at the pipe outlet. Having M2 is very useful in monitoring proximity to sonic velocity, a common problem in compressible flow. 

Valve, injection, 4-way, 0.20 and 0.50 mL injection loop 1 Valve, back pressure, Upchurch 1 Valve, back pressure, relief 1 Valve, flow switching... 

One could think of Example A as being an example of "back pressure," where the more that Valve 1 is closed, the more the pressure tends to escape up through Glass 1. In fact, that is a useful way to remember this little lesson. (Note also that in Example A, the more that Valve 2 is closed, even if only half way, the more pressure tends to escape up through Glass 2, so it works the same way for either valve.) "Back pressure" will turn out to be a useful way to think of "voltage drop," especially in the next chapter. 

Tlie safety valve is similar to the relief valve except it is designed to open fiillv, or pop, with onlv a small amount of pressure over the rated limit. Conventional safety valves are sensitive to dovvmstream pressure and niav have iinsatisfactorv operating characteristics in variable back pressure applications. The balanced safety relief valve is available and minimizes the effect of dovvmstream pressure on performance. 

This condition cannot be corrected on pumps with an enclosed impeller. You need to relax the restricted discharge flow on the pump. The problem could be a clogged downstream filter, a closed discharge valve, an over-pressurized header (back-pressurizing the pump), or a... 

The Back Pressure Regulator (BPR) shown at the end can be a gas dome-loaded Grove Inc. regulator or a spring-loaded Tescom model. The same holds for the forward pressure regulators. Instead of regulators, controllers can be used too, especially since small electronic control valves are now available. 

In most units, the flue gas pressure is reduced to atmospheric pressure across an orifice chamber. The orifice chamber is a vessel containing a series of perforated plates designed to maintain a given back-pressure upstream of the regenerator pressure control valve. 

Another solution is to eliminate the air exiting the unit on the assumption that if no air goes in, no flue gas comes out to drive the expander. Again, this is not very effective because the unit acts as a large surge bottle and the large flue gas valves are relatively slow. Before the flue gas back-pressure valves have acted, excessive acceleration may result. 

The set pressure of a conventional valve is affected by back pressure. The spring setting can be adjusted to compensate for constant back pressure. For a variable back pressure of greater than 10% of the set pressure, it is customary to go to the balanced bellows type which can generally tolerate variable back pressure of up to 40% of set pressure. Table 2 gives standard orifice sizes. 

Case A. The calculated back pressure at the lowest set relief valve on a header is much smaller than its MABP. Reduce header size. 

An oversized relief valve may also chatter since the valve may quickly relieve enough contained fluid to allow the vessel pressure to momentarily fall back to below set pressure only to rapidly increase again. Rapid cycling reduces capacity and is destructive to the valve seat in addition to subjecting all the moving parts in the valve to excessive wear. E.xcessive back pressure can also cause rapid cycling as discussed above. 

Increase the absorber pressure, if needed. This may require the installation of a back pressure valve. 

For the tandem arrangement gas seal, a primary seal vent must be pro vided to vent the leakage across the process side seal. This vent ma> lie to flare or other suitable gas disposal point. The back pressure under nor mal conditions should be kept to a low value. A small amount of back pressure is recommended to keep a positive differential across the see ondaiy seal. Leakage measurement may be provided in the vent line to provide health monitoring of the primary seal. Unfortunately, the rotameter, which would be the obvious choice, should not be used because of its lack o reliability. If an orifice or needle valve is used to set the back pressure to the seal vent, pressure upstream of the restriction can be measured for a relative flow measurement. This type of reading does provide trend data that may be used to judge the seal s performance. 

Steam turbine, 53, 146, 282-92, 179 back pressure, 282 blade deposits, 479 condensing, 282 efficiency, 288 extraction, 282 induction-type, 282 paitial admission, 288 rating, 290 reliability, 478 selecuon variable, 275, 285 speed, 278 stage losses, 286 steam temperatures, 284 steam velocity, 288 trip and throttle valve. 479 Step unloading system, 80 Stiffness coefficients, 385 Stodola slip, 153, 155 Stonewall, 186 Straight labyrinth. seal leakage, 532... 

Safety Relief Valve - A safety relief valve is an automatic pressure-relieving device suitable for use as either a safety valve or relief valve, depending on application. (In the petroleum industry it is normally used in gas and vapor service or for liquid.) Safety relief valves are classified as "Conventional" or "Balanced", depending upon the effect of back pressure on their performance. 

Conventional Safety Relief Valve - A conventional safety relief valve is a closed-bonnet pressure relief valve that has the bonnet vented to the discharge side of the valve and is therefore unbalanced. The performance characteristics, i.e., opening pressure, closing pressure, lift and relieving capacity, are directly affected by changes of the back pressure on the valve. 

Cold Differential Test Pressure - The cold differential test pressure (in kPa gage) is the pressure at which the valve is adjusted to open on the test stand. This cold differential test pressure includes the corrections for service conditions of back pressure and/or temperature. 

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. 

Superimposed Back Pressure - Is the pressure at the outlet of the pressure relief valve while the valve is in a closed position. This type of back pressure comes from other sources in the discharge system it may be constant or variable and it may govern whether a conventional or balanced bellows valve should be used in specific applications. 

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. 

Spring Pressure - The spring pressure is equal to the set pressure minus the superimposed back pressure for a conventional PR valve. For a balanced bellows safety relief valve, the spring pressure equals the set pressure. 

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 operation and characteristics of a conventional safety relief valve are illustrated diagrammatically in Figure 3. The action of the valve as pressure rises from the initial normal operating pressure (assuming no back pressure) is described below. The effect of back pressure on PR valve operation is described later. 

Superimposed back pressures are not constant. (Where back pressures fluctuate on a conventional valve, the valve may open at too low a pressure or permit the vessel pressure to exceed the equipment rating, depending upon back pressure fluctuation.)... 

In addition to the above back pressure limitations based on valve capacity, balanced bellows PR valves are also subject to back pressure limitations based on the mechanical strength of the bellows or bellows bonnet, or the valve outlet flange rating. The back pressure specified for the valve is governed by the lowest back pressure permitted by these various criteria. 

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. 

Pilot-operated valves may be satisfactorily used in vapor or liquid services up to a maximum back pressure (superimposed plus built-up) of 50% of set pressure, provided that the back pressure is incorporated into the sizing calculation. At higher back pressures, capacity becomes increasingly sensitive to small changes in back pressure. As an exception, back pressure up to 7% of set pressure may be used, provided that this disadvantage is recognized. 

For applications involving unusually high superimposed back pressure, a pilot operated valve may be the only possible balanced valve that is commercially available, because of the mechanical limitations which apply to bellows. 

Back pressure reduces the pressure drop across the orifice of any type of PR valve. This results in reduced discharge rates in the case of vapors, if the back pressure exceeds the critical flow pressure. For liquids, any back pressure reduces the pressure drop and results in a lower discharge rate. 

A balanced bellows PR valve is one in which the closing force exerted by back pressure on the top of the valve disc and the back pressure exerted on the bottom of the disc are balanced by equal surface exposure, thus cancelling each other. The bellows shields the top of the disc from back pressure and the bellows area is vented to the atmosphere via the bonnet vent. 

In the case of a pilot-operated valve, provided that the pilot valve exhausts to the atmosphere, the main piston is independent of back pressure and is thus also considered as a balanced valve. Balanced PR valves can be characterized by the following ... 

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