Operation spring valves

Pilot-operated relief valves use the pressure in the vessel rather than a spring to seal the valve and a pilot to activate the mechanism. Figure 13-6 is a schematic of a typical pilot-operated valve. A piece of tubing communicates pressure between the relief valve inlet and pilot. When this pressure is below the set pressure of the pilot, the pilot valve is in the position... 

A condenser pressure regulator can be in the form of a pressure-operated bleed valve in a bypass across the condenser, to divert hot gas to the receiver. The valve diaphragm is balanced by a pre-set spring and will open the bypass if the condensing pressure falls. A similar effect can be obtained by a pressure-operated valve between the condenser and the receiver, to restrict the flow and allow liquid to accumulate in the condenser, reducing its efficiency. For operating... 

Directly actuated valves weight or spring-loaded valves that open at a predetermined pressure, and which normally close after the pressure has been relieved. The system pressure provides the motive power to operate the valve. 

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). 

Fig. 7.119. Pneumatically operated control valve (a) double-spring actuator with single-ported globe valve (b) exterior view of double-ported control valve with valve-positioner fitted on the side...

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). 

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... 

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. 

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. 

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... 

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. 

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

In extreme cases, some spring valve models can perform with higher backpressures if a pilot-operated valve is absolutely not acceptable. 

The liquid capacity is >30% total (volumetric capacity) If a spring valve is required, it should be a valve with liquid trim. Preferably, however, a modulating pilot-operated valve can be used. The modulating pilot ensures stability of operation whatever the phase and cannot be oversized as it adapts its flow to the need of the system. 

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. 

When both of the above solutions are too much of an investment, we will have to go to in situ testing of the valves, which is also for spring-operated valves sometimes a cumbersome and expensive solution. Different service organizations offer the in situ testing on spring valves. Most are based on the same principle where set pressure is actually calculated based upon an external force applied on the spring (hydraulic or pneumatic). 

The functioning of a spring valve is based on a pre-stressed spring. When the pre-stress force is reached, the disc starts to move and the valve opens. With lifting equipment attached to the valve spindle, the valve is opened and the pre-stress force measured, taking into account the above factors (seat area and operating pressure). 

Pressure-relief devices include direct spring-loaded and pilot-operated relief valves that close or reseat if the vessel pressure is brought back into a safe condition, as well as non-reclosing devices such as rupture disks and breaking-pin devices. 

The Easi-Breathe is a breath-actuated metered-dose aerosol inhaler developed by Norton Healthcare. The device is primed when the mouthpiece is opened. When the patient breathes in, the mechanism is triggered and a dose is automatically released into the airstream. The inhaler works on a pneumatic principle. An internal vacuum restrains an operating spring. A valve that is operated in response to the patient s inhalation that allows the spring to fire the canister releases the vacuum. It can be actuated at a very low flow of approximately 20 L/min (72). Not surprisingly the device scored better than a pMDI on a number of features, including ease of use and having an attached mouthpiece cover. Practice nurses found it easier to teach and patients easier to learn to use than a conventional pMDI (73). 

Raising air is admitted by a norm ally-closed (closed when de-energized) solenoid valve located at the bottom of the cylinder, along with a solenoid operated,normally-open bleed valve and orifice and a spring operated relief valve. A large, normally open solenoid exhaust valve with an orifice is... 

Under normal operation, the valve disk is kept in the closed position by the delivery pressure of the MCP against the spring force. The valves open passively due to loss of the delivery pressure of the MCP in case of MCP failure. If the delivery pressure of the MCP is lost, the disk is pushed back to open position by the spring force. Borated water in the poison tanks has enough ability to maintain a cold shutdown condition. 

Each steam line has two containment isolation valves, one inside and one outside the containment barrier. The isolation valves are spring-loaded pneumatic piston-operated globe valves designed to fail closed on loss of pneumatic pressure or loss of power to the pilot valves. Each valve has an air accumulator to assist in the closiue of the valve upon loss of the air supply, electrical power to the pilot valves, and failure of the loaded spring. Each valve has an independent position switch initiating a signal into the reactor protection system scram trip circuit when the valve closes. 

Shutoff nozzles are used to prevent nozzles from leaking into empty molds or when the nozzle is retracted from the sprue bushings. Valves of all kinds —i.e., spring-loaded ball check valves, hydraulically and mechanically operated needle valves, and rotary valves —have been incorporated into nozzles. 

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