Soft seat

Seat leakage is specified for conventional direct spring operated metal-to-metal seated valves by API RP 527. The important factor in understanding the allowable seat leak is that it is stated at 90% of set point. Therefore, unless special seat lapping is specified or soft seat designs used, a valve operating with a 10% differential between operating and set pressures may be expected to leak. 

The soft-seated spring-loaded pilot valve is so constructed as to have a long built-in blowdown. For a flowing type pilot, at the point where the pilot supply line feeds the system pressure to the pilot relief valve, it passes through a variable orifice, which is also the main valve blowdown adjustment. When the pilot opens, the flow through the supply line causes an immediate pressure drop across the orifice. By adjusting the size of the orifice and thus the amount of pressure drop across it, one can obtain any desired system blowdown (5 to 7% is typical). 

Valve Body Temperature 705 °C (1300 °F) (Flame) Not Mentioned Able to destroy the soft seat Minimum 650 °C (1200 °F) Minimum 650 °C (1200 °F)... 

American Petroleum Institute (API), Std 607. Fire Test for Soft-Seated Quarter Turn Valves. Fourth Edition, API, Washington, D.C., 1993. 

Fire Test for Soft Seated Quarter Turn Valves, 1993. 

The use of soft seat service line valves is not recommended when the design of the valves is such that exposure to excessive heat could adversely affect the ability of the valve to prevent the flow of hydrogen gas. 

Both floating and trunnion-mounted designs are available with other design variations that include metal seied valves, soft seated valves, top-entry valves, end-entry valves, and split body valves. Valves in all these design variations are available as either full port or reduced port. Port refers to the round-bore fluid flow area through the ball. Full port valves have a bore that is approximately equal to the inside diameter of the mating pipe. Reduced port valves have a bore that is approximately equal to the inside diameter of pipe one size smaller than full bore. 

A variety of soft seat materials are available, including PTFE and nylon. Since the shutoff pressure capability of ball valves is mnited by the load capabilities of the seat material, the upper temperature limit of soft seated valves is limited by the seat materim selection. The shutoff pressure rating of soft seated valves typically declines rapidly with increasing temperature, and the shutoff rating is often less than the body pressure rating. Metal seated valves do not are this characteristic. 

API Standard 527, Seat Tightness of Pressure Relief Valves This standard describes tests with air, steam and water to determine the seat tightness of metal- and soft-seated PRVs. Valves of conventional, bellows- and pilot-operated designs are covered. Acceptable leakage rates are defined for gas, steam and liquid. 

To improve the sealing characteristics of small orifice valves, the seat sealing area is sometimes made larger than the through bore For soft-seated valves, a softer elastomer is also used as long as the temperatures are not too high. Here the manufacturer has got to find the correct compromise. 

Soft seat leakage at 90% of set pressure in bubbles/minute after 1 pop ... 

Most discs can also be provided with soft seats for more frequent and trouble-free operation, but this design is limited by temperature. How the soft seat is fixed to the disc differs from manufacturer to manufacturer, but here two factors are important First, the seat must be easy to replace, but the design must also be such that the soft seat cannot be blown out during a relief cycle (Figure 5.14). 

Specific designs also affect how easily discs can be converted from metal- to soft-seated designs. 

Optimum tightness up to set pressure with soft-seated designs... 

Soft-seated, leak-free and snap-acting valves are also recommended for cryogenic applications as the typical leakages allowed on metal-to-metal API valves could cause icing of the seat area, which could cause the valve to fail. 

Unfortunately, a lot of the operational objectives for the POSRV described above could not be achieved as economically as with a spring valve the use of soft seats was imperative to obtain all advantages, which limits the use of most POSRVs in high temperatures (typically up to 300°C maximum). There are now POSRVs in the market with metal-to-metal seats, but here tightness, especially after a few operations, decreases much more than with resilient-seated valves and even traditional spring-operated metal-to-metal valves. 

The advantage of soft seats is also that they are very suitable for cryogenic applications. 

Operating pressures between 90% and 95% of set Here the use of pilot-operated safety relief valves (POSRVs) or soft-seated, spring-operated SRVs should be considered. Metal-seated spring valves will not stay tight for long and usually get damaged after a couple of operations. On the other hand, soft-seated valves are limited in temperature and sometimes also in pressure (see Section 5.2.6.5). 

Operating pressures above 95% of set Preferably use modulating POSRVs or soft-seated valves. The choice for the more expensive modulating pilots depends on system pressure fluctuations. The more fluctuation dose to or over set pressure, the more a modulating valve should be considered. 

Short blowdown necessary outside ASME I or VIII requirements (<7%) Some POSRVs or high-performance, soft-seated, spring-operated SRVs have large adjustable blowdown ranges. Some range from 3% to 30% blowdown adjustment. This is, however, only available with a limited number of suppliers. A normal standard ASME VIII spring-operated SRV may not reach full lift at 10% overpressure when it is adjusted for a very short blowdown. Blowdown can seldom be set shorter than 5% on conventional spring valves. 

C and below Here, it is highly recommended to use soft-seated, pop action POSRVs or soft-seated, high-performance, snap action spring valves which operate beyond the ASME and PED recommendations. Look for a valve... 

Below -70°C and thermal expansion Typical applications here are LNG, LIN and LOX. Here again, we must avoid the risk of freezing, but in this case we also need to consider repeatable tightness therefore, high-performance, soft-seated spring valves should be used. A metal seat may leak prematurely, freeze up and start acting like a blind flange. 

Below -70°C and backpressures needing a balanced valve Pilot-operated, soft-seated valves are the only valid solution here. Balanced bellows valves are not acceptable at these temperatures due to the high risk of bellows failure ( cold working) and the risk of condensates freezing inside the bellows. This phenomenon is discussed in detail in Chapter 11. 

Below —70°C and no backpressures Pilot-operated valves are still preferred, but spring valves could be used, preferably soft-seated ones. It should be noted that metal-seated valves always present a high risk of freezing at these temperatures. 

Fully cryogenic licjuid service Pilot-operated, soft-seated valves with a type of vaporizer which (relatively) warms up the fluid entering the pilot or high-performance, soft-seated, spring-operated SRVs. The vaporizer and other accessories of a typical cryogenic configuration on a pilot-operated valve keep the pilot warm, which then works on vapour. In any case, these are applications that should be discussed with your SRV supplier. Some suppliers have done extensive tests on cryogenic applications and have experience to share on this specific application. 

Dirty service with particles in the fluid A spring-loaded valve, preferably with elastomer soft seat is preferred (preferably no PTFE). An elastomer soft seat greatly limits seat damage. PTFE is hard and easily scratched and not good for particle-laden fluids. If a pilot-operated valve is preferred or necessary for the application, the pilot should be protected from the particles. There are different options and configurations to protect pilot valves nowadays, but the option depends on the dirtiness of the fluid. This should be discussed in detail with the SRV vendor (also see Section 5.3.3.4). 

Although lacking mathematical proof, I have nonetheless experienced that a soft-seated valve needs about 2.5 times less maintenance than a metal-seated valve This is logical considering the higher possible operating frequency of a... 

A soft-seated pilot-operated valve spare part kit ... 

Retrofit valve to soft-seated or high performance design. 

Particles (or traces thereof) between seat and disc - especially frequent in pump applications 1. Use a harder and different seat/disc combination. 2. Make sure a knife = edged seat arrangement is used. 3. Use easy to retrofit and compatible soft-seat design - preferably O-ring design. 4. Use bellow valves in case large particles are found or use a rupture disc at both valve inlet and outlet. 

There are very special resilient-seated cryogenic SRVs on the market (some initially developed for NASA), bubble tight up to 98% of set pressure, with a snap opening at 101% of set pressure and adjustable blowdowns between 3% and 25%. If you do not want to spend the money for these very special valves, then at least consider a soft-seated relief valve with a snap action. 

Furthermore, a correctly designed soft seat usually will guarantee a good repeatability of this tightness, even after many cycles of operation when metal-seated valves have already lost their tightness because of the repeated hammering of the two metal parts against each other. 

However, with many manufacturers, all the attempts to use special techniques in the pilot without the use of soft seats have so far proved unsuccessful after testing (erratic set and reseat pressures at least). So, as of today, the use of O-rings in the pilot is unavoidable, and therefore the pilot must be kept away from low temperatures as much as possible. 

While conventional spring-loaded SRVs can provide an acceptable level of reliability, particularly when fitted with a soft seat of proven design, the balanced bellows spring-loaded SRV can create a highly hazardous situation and should therefore be avoided on cold or cryogenic services. 

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