Customer question: How does a back-pressure-independent safety valve work

Hansa Kältetechnik: The term ‘back-pressure-independent’ refers to the opening behaviour of the safety valve; however, it should be noted that whilst the valve opens when the pressure exceeds the opening pressure, it can only vent gas once any existing back pressure (from the refrigeration circuit) has been exceeded.

A spring-loaded valve operates in such a way that it opens slowly as the pressure rises. As a result very little gas is vented if the pressure is only slightly exceeded, meaning that any back pressure in the system is not exceeded. For this reason a check valve should be installed in the vent line leading to the low-pressure section of the system. This ensures that no back pressure is present at the valve, and thus even a slight, brief exceedance of the response pressure leads to the safe discharge of the excess pressure.
For safety valves in general, a permissible back pressure of 10% of the set pressure is assumed to ensure safe operation; this also applies to Hansa ÜSV valves.

The following factors may prevent the valve from opening when the set pressure is exceeded (tolerance: 3% of the specified set pressure):

1.)  Tampering with the valve (only possible if the sealed cap is removed).
2.)  Incorrect installation of the valve, for example, the valve is soldered in place in addition to being screwed in.
3.)  Additional securing of the valve using a tension strap over the cap, preventing the spring from actuating.
4.)  Use of assembly adhesive at the valve inlet; this could cause the valve seat and the elastomer to stick together upon contact.
5.)  Blockage of the valve’s blow-off outlet.

Contamination on the inlet side does not usually prevent the valve from opening, but can result in
the valve no longer closing tightly, e.g. when sealing the valve inlet with Teflon tape or hemp, which settles between the seat and the elastomer when the valve opens. In rare cases, brass or copper shavings may also settle, or other deposits and oil may be blown onto the sealing disc.