Valves spring-operated

Solenoid Valves The electric solenoid valve has tw o output states. Wlien sufficient electric current is supplied to the coil, an internal armature moves against a spring to an extreme position. This motion causes an attached pneumatic or hvdraiilic valve to operate. Wlien current is removed, the spring returns the armature and the attached solenoid valve to the deenergized position. An intermediate pilot stage is sometimes used when additional force is required to operate the main solenoid valve. Generallv, solenoid valves are used to pressurize or vent the actuator casing for on/off control-valve application and safetv shutdown applications. 

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 face of the wear plate should be checked periodically and remachined if necessary. A whistling noise during operation is an indication the wear plate Is worn out or the valve spring requires tensioning. 

Kj = valve coefficient of discharge (from valve manufacturer) Farris and Consolidated spring-operated,... 

There are two general categories of relief devices (spring-operated and rupture discs) and two major types of spring-operated valves (conventional and balanced-bellows), as illustrated in Figure 8-7. 

On spring-operated valves the adjustable spring tension offsets the inlet pressure. The relief set pressure is usually specified at 10% above the normal operating pressure. To avoid the possibility of an unauthorized person changing this setting, the adjustable screw is covered with a threaded cap. 

For a conventional spring-operated relief, the valve opens based on the pressure drop across the valve seat that is, the set pressure is proportional to the pressure drop across the seat. Thus, if the backpressure downstream of the valve increases, the set pressure will increase and the valve may not open at the correct pressure. In addition, the flow through the conventional relief is proportional to the difference in pressure across the seat. The flow through the relief, therefore, is reduced as the backpressure increases. 

Rupture discs are available in much larger sizes than spring-operated relief valves, with commercial sizes available up to several feet in diameter. Rupture discs typically cost less than equivalently sized spring-operated relief valves. 

Describe the advantages and disadvantages of a rupture disc followed by a spring-operated relief valve. See Grossel and Louvar (2000). 

First open-ended spring-operated Safety Relief Valve... 

Some companies or local customs require the valves to be tested just before start-up, but normally the valves have already been set and sealed correctly at the manufacturer. This is not a recommended practice, but if needed on spring-operated valves, crack pressure can be checked by applying a suitable pressure source at the inlet of the valve. However, in the usual case on site, the volumetric capacity upstream is insufficient, and therefore a false reseat pressure (usually lower than actual) will be obtained. 

Constant superimposed backpressure Usually backpressures that occur when a safety valve outlet is connected to a static pressure source and doesn t change appreciably under any conditions of operation. In this case, conventional valves may be used if the valve spring setting is reduced by the amount of the constant backpressure (Figure 3.8). 

Power actuated/assisted safety valves (CSPRS - controlled safety pressure relief system) A spring-operated safety valve actuated or assisted by an externally powered control device which can be hydraulic, pneumatic or electric (Figure 3.15). 

API Standard 526, Flanged Steel Pressure Relief Valves This is a purchase specification for flanged steel PRVs. Basic requirements, such as orifice designations and area, materials, pressure-temperature limits and centre-to-face dimensions, inlet and outlet, are given for both spring-operated and pilot-operated PRVs (Figure 4.13). 

However, since 1985, the code has also required a maximum overpressure of also 10% on liquid valves. This meant manufacturers had to redesign many trims so that the same valve could be fitted for both gas and liquid. So it is important to know the age of a liquid valve in order to determine whether it will flow full capacity at 10% or at 25%. Also, blowdown on liquid spring-operated valves was and is still rather unstable, and can be as high as 30% depending on the design. The code does not specify a blowdown requirement for liquid valves. 

Disc (disc insert) A metal disc with (in the case of a metal-to-metal, spring-operated relief valve) a smoothly lapped surface which ensures tightness in... 

Types of spring-operated SRVs 5.2.6.1 Thermal relief valves... 

In order to overcome the various problems encountered when using spring-operated SRVs and also at NASA s request, the industry started looking for ideal SRV characteristics and tried to design a valve that came as close as possible to these characteristics. Thus, the POSRV was bom. 

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. 

Spring-operated metal-seated safety valve... 

If there is a risk of blockage, this risk should be eliminated and that applies to a spring-operated valve as well as for a pilot-operated valve. Some manufacturers have acted on this and can propose many different accessories and configurations to achieve this which have been proven to work. Pilot valves can be protected against dirt with a variety of options, and most spring valve suppliers can supply such items as steam or electrical jackets, to avoid, for instance, polymerization or formation of hydrates. 

There are a very limited number of good solutions available on the market, and what there are should be carefully discussed with the manufacturer. Here, the recommendation would be to use a spring-operated valve whenever possible... 

Extreme fast-acting pressure relief is required Rupture discs can be considered as process pressure protection when there is a potential risk for chemical runaway reactions. Depending on the choice of the SRV (for instance, a non-coded spring-operated valve), there could be a risk that they would... 

Rupture disc 3 in. Hastelloy C discs = 2.500 for first usable disc, then 850 per disc plus a 3in. Hastelloy C holder = 3.100 ea. Total for 3 pair 13.500, versus a Hastelloy C 3 in. X 4 in. standard spring-operated valve for around 40.000. 

In particular, vibrations and pulsations due to positive-displacement compressors can cause premature opening when the forces within the valve are anywhere near equilibrium and are acting as another upward force. For pilot-operated valves, some manufacturers provide pulsation dampeners in their pressure pick up lines so that the effect of these pressure spikes are compensated. As can be seen in Figure 6.21, for spring-operated SRVs, unfortunately, no provisions can be taken to compensate for this effect. 

However, a conservative approach, which has been successful for many years in my personal experience, is to limit the liquid phase to about one-third by volume. Anything above that will need at least a liquid trim for a spring-operated valve or a modulating pilot as a solution to this problem. Oil/gas separators are a typical application where this rule of thumb can be applied. 

These guidelines will provide some guidance in selecting a safety relief valve (SRV) for use in a specific application or process condition where the use of traditional spring-operated valves could be questionable. 

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). 

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... 

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. 

Value (% of set) Conventional Balanced spring valve Pilot-Operated ... 

Normal operation Spring-operated valves can be used but need to be equipped with a trim suitable for liquid service so they can obtain nominal flow at 10% overpressure. When quick opening (and closing) is required, the operation will be unstable and will possibly cause water hammer. Alternatively, a modulating pilot valve can be used, preferably equipped with a filter. The volume of the filter slows the pilot, which may otherwise react too fast, creating instability and water hammer. 

Another extremely important cost factor is the weight of the valve and especially where it is installed when it has to be removed for testing and maintenance SRVs are usually mounted high up. Therefore it is important to take weight and size into consideration when selecting a valve in order to keep down later maintenance costs. A pilot-operated safety valve is much lighter than a spring-operated valve from sizes 3 to 4 in. and up and can more easily be handled. 

In case of pilot valves check for any obstructions in the supply piping to the pilot or clogged filters Remove obstructions and clean filters. Change to a spring-operated valve. 

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