﻿ Spring Formula

The Use Of Ready Wound Springs In Ordinary (non pop) Safety Valves. by David Hudson.

The formula for calculating the area of one safety valve, based upon a clear unobstructed opening equal to the bore and, derived from the formula for a Ramsbottom type safety valve is given by:-

"A". in sq.ins = total boiler heating area in square inches

Alternatively,

The diameter “D”. In inches =  Total boiler heating area in square inches

If two safety valves are required, the area of each will be “A” X 0.5 and the diameter of each will be “D” X 0.707 also, if four safety valves are required the area of each will be “A”  0.25 and, the diameter of each will be  “D” X 0.5.

The spring force required to balance the boiler pressure may be expressed as:-

“F” . In Lbs.= Area “A” in square. Inches X boiler pressure  “P” in P.S.I.

if we assume that a valve must be allowed to lift by 10% of “D” with an increase in boiler pressure of 10%, then the maximum spring rate allowed can be determined as followers:-

Max spring rate  “S.R” =

Messres  A.j.Reeves, let me have samples of three grades of stainless steel springs in three inch lengths for cutting to size. Tests carried out on these samples have yields the following information:-

24 g.  14 T.P.I.     D/d ratio = 8.14  “S.S.R.  =64.2 Lbs/inch/turn.

22g.   12.5 T.P.I.  D/d ratio = 6.64  “S.S.R.” = 156 Lbs/inch/turn.

20g.   11.5 T.P.I.  D/d ratio = 5.69  “S.S.R.” = 314 Lbs/inch/turn.

Note;- Sample springs were fully compressed for twenty four hours before cutting to size and testing. This procedure should be followed before cutting springs to size.

Now the actural spring rate “S.R.” In Lbs/inch. Is given by:-

“S.R.” =

When expressed in terms of “N”. This becomes:-

Minimum number of turns “N” =               =

Note:- If springs do not have closed coil and ground ends then, “N” = the total number of turns, and should never be less than five when using these D/d ratio.

The minimum free length maybe calculated using :-

“L”  =

The yield the required, spring force “F”. The spring should be compressed by:-

“C”  =

To check that the right choice of spring has been made, use the following :-

Ideally this should give a figure below 25% using these D/d ratios, this recommended figure is to keep the stresses within acceptable limits and to prevent undue fatigue due to continuous stress and creep.

The Use Of Ready Wound Springs In Ordinary (non pop) Safety Valves. by David Hudson.                                            Table Of Springs