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Absolutely tight to the outside

Low-emission safety valves
Absolutely tight to the outside

Industry safety valves are used for many different applications and with a host of different media. They form an intended interface to the outside and are of special significance for plant tightness. Selecting the proper safety valve can contribute to reducing unwanted emissions.

The author: Julian Witt Product Management, Leser

Distinguishing between tightness towards the outside and functional seat tightness is an important part of selecting a safety valve. Shell tightness defines a valve’s tightness in relation to the environment (tightness to the outside). Functional tightness refers to the tightness between the valve’s seat and disc. In the case of shell tightness TA-Luft (German Air Quality Control Guidelines) must be considered. It refers indirectly to safety valves and requires that “when sealing spindle feedthroughs for closing or control devices … high-grade sealed metal bellows with a subsequent safety gland or equivalent sealing system” be used.
Improved body tightness
With Leser safety valves, TA Luft requirements are met through the flange coupling and the screw connection. Valves with a closed body and cap have no additional spindle feedthrough to the outside as is the case with other fittings. Regarding flange connections TA Luft stipulates that “compliance with a specific leakage rate of 10-4 mbar l/s … be shown through a design test pursuant to Directive VDI 2440”. Safety valves have a flange connection between the body and the bonnet. To comply with the required leakage rate, valve manufacturers must perform tests to establish conformity. On models with a packed lever the tightness of the spindle feedthrough (in this case the shaft feedthrough) is ensured by a standard stainless steel bellow that conforms to TA Luft.
In the chemical industry, the use of toxic substances (e. g. phosgene) places increased demands on tightness towards the outside beyond those of TA Luft. In cooperation with a large German chemical company, Leser has developed a configuration for toxic applications that provides a tightness of 10-6 mbar l/s – one hundred times higher than TA Luft. This valve has a stainless steel bellow, a special sealing concept and additional elastomer seals. The configuration is equipped with a pressure monitoring unit and subjected to a standard helium leak test prior to delivery. This Toxic Service design is employed for toxic media, for example in HDI and TDI plants which manufacture intermediate materials for plastic production.
Supplementary loading system
The tightness between a valve’s seat and disc is the process-specific tightness of a safety valve. Spring loaded safety valves are employed at an operating pressure equivalent to 85 or 90 % of the set pressure. The closer the operating pressure comes to the set pressure, the lower the closing force of the valve. API 527 is the only internationally recognised standard for safety valve tightness, including requirements for leakage rate and testing procedures, that insists on functional seat tightness test at 90 % of the set pressure. All Leser safety valves are delivered with tightness according to API 527 as a standard feature. Whether test pressure should be applied starting at zero pressure (0 bar) or from the set pressure downwards (10 % ptest = 0.9 Δptest) is not defined. Leser performs its tests using the more stringent method from the set pressure downwards. This way, settling of the disk after blowdown is prevented.
A further improvement in safety valve tightness can be achieved in conjunction with a supplementary loading system. This system increases seat tightness by applying additional pressure to the valve’s disk and seat as long as the valve is closed. Another option for achieving better seat tightness is the pilot operated safety valve (POSV), which is still relatively unknown in Europe. The closing force in the POSV is provided by routing the fluid into the dome, so that the closing pressure increases as the operating pressure approaches the set pres-sure. This allows the operating pressure for gas plants to be run at 97 % of the set pressure, which often leads to higher plant efficiency.
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