Spring valves

Some questions will require the identification of various mechanical tools or devices. Some of the types of mechanical devices that may appear on the exam— and covered in this chapter—include hand tools, gears, pulleys, levers, fasteners, springs, valves, gauges, and pumps. In addition to individual mechanical devices, the exam may test your knowledge of various systems, or combinations of mechanical devices. A common example of a mechanical system is the internal combustion engine of an automobile. 

Valve Seats Valve Springs Valve Stops Valve Plates... 

Typical spring values fora full nozzle 3K4 spring valve... 

The basic design of a balanced bellows spring valve is the same as a conventional valve, but a bellows is added to compensate for variable backpressures and the bonnet is vented to atmosphere A leak at the bonnet vent indicates a failed bellows. Because balanced bellows valves must have their bonnets vented to the atmosphere, a safe location for piping the vent must be determined. The balanced bellows valve, with or without a supplementary balancing piston, only discharges process medium from the bonnet vent in the case of failure of the bellows (Figure 5.26). 

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 graph in Figure 5.37 shows the product loss comparison for a spring valve, a pop/snap action pilot valve and a modulating valve. It is obvious that the product gain of a modulating valve is important (Figure 5.38). 

Some mechanical tests on spring valves are not always accurate some systems can be satisfactory but remain rather cumbersome and expensive, as explained in Section 10.4. They require a very accurate knowledge of the system pressure, the nozzle area, spring compression, and so on. During some tests, the valve always opens, which is not recommended as it can damage the nozzle and disc in case of a metal-to-metal valve. It is also very time-consuming. 

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 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 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. 

Too large an orifice recjuired for the available connection size Some manufacturers can offer full-bore POSRVs where a larger orifice can be obtained in a smaller size valve by increasing the curtain area. Also some spring valves can be equipped with customized nozzles. 

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. 

Rating and Size 2" x 3" - 600 Spring Valve (cm) 58 Pilot Valve (cm) 48 Height Saving (%) 17... 

Rating and size 2" x 3" - 600 Spring valve (kg) 31 Pilot valve (kg) 24 Weight saving (%) 23... 

Some of the following recommendations relate to both spring valves and pilot valves, while others only apply to pilot valves. 

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). 

Because of the uncertain reliability of balanced bellows spring valves on cold and cryogenic services, POSRVs have offered a good alternative for these applications for more than 35 years now. 

Flanged spring valves shall be of full nozzle design, arranged as such that the nozzle and the parts comprising the disc are the only parts exposed to the inlet pressure or to the corrosive action of the inlet fluid when the valve is closed. 

Precipitation hardening stainless steels are those alloys ending with the sufBx PH (i.e., precipitation hardening), for example, 17 PH (17Cr-4Ni-4Cu) (UNS S17400). They are hardenable by heat treatment. They are most often used for springs, valve stems, the internals of rotating equipment. 

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