Built up backpressure

Safety Relief Valves Conventional safety relier valves (Fig. 26-14) are used in systems where built-up backpressures typically do not exceed 10 percent of the set pressure. The spring setting or the valve is reduced by the amount of superimposed backpressure expecied. Higher built-up backpressures can result in a complete loss of continuous valve capacity. The designer must examine the effects of other relieving devices connected to a common header on the performance of each valve. Some mechanical considerations of conventional relief valves are presented in the ASME code however, the manufacturer should be consulted for specific details. 

Backpressure The static pressure which exists at the outlet of an SRV due to existing pressure in the discharge system. It is the sum of superimposed and built-up backpressure, and potentially influences the set pressure and certainly the operation of the valve (Figure 3.6). 

Built-up backpressure Occurs when the safety valve is open and flowing due to ... 

Built-up backpressure is always variable and usually occurs due to friction (vortices/turbulences) and pressure drops through the discharge piping. It is allowed to be up to 10% (of set pressure) on conventional design SRVs but will cause reduced capacity and unstable operation if the pressure gets over 10%. Therefore, if the built-up backpressure is greater than 10%, we will need to use a balanced bellows - or pilot-operated design SRV. 

This illustrates why a conventional SRV is best suited for simple discharge via a tail pipe into atmosphere. Its ability to tolerate built-up backpressure is very limited. 

Conventional SRVs are normally used in any services where the superimposed backpressure is constant and/or the built-up backpressure does not exceed 10% of the set pressure. 

In cases where the built-up backpressure exceeds 10% of the set pressure... 

When piping the outlet to headers, flash drums, scrubbers, and so on, it is highly recommended to diverge at least one size up from the outlet piping to allow the fluids to expand this avoids unnecessary built-up backpressure and turbulences in the outlet, which creates instability in the valve. This is basically the same reason the outlet of an SRV is always one size up from the inlet (Figure 6.16). 

When discharging to atmosphere via a vertical-rising tail pipe, recommended avoid using a flat-ended tail pipe, as shown in Figure 6.17. It is best to provide a 45 ° angle ending to the tail pipe to reduce built-up backpressure, reaction forces and noise while discharging. 

One of the main concerns in closed systems is the built-up backpressure in the discharge system as this can drastically affect the performance of an SRV. 

While here the tail pipe has the right construction and orientation, it is unfortunately not equipped with a drain. Therefore dirt can accumulate and create excessive backpressure, and can enter the valve and damage the seat surface, causing valve leakage after operation. The size of the tail pipe at the connection is smaller than the valve outlet. This will provide excessive built-up backpressure. Again, these considerations must be taken into account when sizing the valve, as the valve may otherwise be undersized. ... 

The oudet of the tail pipe shown in Figure 6.30 is directed downwards, which will not only create excessive built-up backpressure but is also not safe for personnel and equipment. The flow of an opening SRV can achieve high velocities and can be toxic, hot and flammable. It is highly recommended that the oudet be directed to the sky. ... 

The oudet of the tail pipe shown in Figure 6.31 is far too near the walking deck (almost under the walking surface). It would be inadvisable to walk on this deck when this SRV goes off. When discharging, it will blow against the deck, which can possibly create built-up backpressure. ... 

It is difficult to account for hysteresis in a second-order model because of its nonlinearity with viscous damping (disk movement in the relieving fluid), its lack of repeatability, and its dependence on built-up backpressure (and hence the particular application). However, where additional viscous damping is deliberately added, a linearized analysis will yield useful results. ... 

Dual outlets are used to minimize built-up backpressure. They are usually used in high-pressure protection and where the maximum use of the bore is... 

Comment Conventional safety relief valves are for applications where excessive variable or built-up backpressure is not present in the system into which the valve discharges (Fig. 5.191). 

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