What is "Free length" of springs in safety relief vavles??

[kokerkov]

It sounds silly. But it has a great effect on our product.

Our springs cannot meet the requirement of “ASME BPVC 2023 Section XIII”, Part 3.2.5, which is :

(b) The permanent set of the spring shall not exceed 0.5% of the original free length. Permanent set is defined as the difference between the original free length and the free length after the spring has been preset at room temperature by compressing it to its solid height threetimes. Measurement shall be taken at least 10 min following the preset.

Where “original free length” is not defined, unlike ISO 4126-7：

The permanent set of the spring is defined as the change in the spring's free length as a result of a series of compression cycles to solid in accordance with the safety valve manufacturer's specification or other appropriate standard. The spring shall be compressed to solid at least three times before determining the initial free length. The spring shall then be compressed to solid at least three more times before measuring the final free length. The permanent set shall not exceed 0,5 % of the initial free length.

Our spring can manage to meet the ISO test, but our client said ASME test's "orignial free length" should be tested as springs are delivered. So anyone who are familiar with ASME can give us an explanation? Our best guess is from ASTM A125, which free length is defined as "length after test load is removed", and "test load" is "solid load if possible".

Thanks in advance!

 

[LittleInch]

I really can't see any difference here.

both say "original free length".

When you measure Original free length is surely up to you?

Basically they are just making sure you don't yield the spring when you compress it to solid / cant compress it any more....