Spring Adjustment in Pressure relief valve

[Channo]

Hello all,

I read that in pressure relief valves(hydraulics), there is knob, which can be adjusted to increase the spring tension, which in turn increases the relief pressure(pressure which pushes the spring and opens a path to tank).

This spring tension depends upon the spring constant, which (by the name itself) is constant.

So, when the spring constant remains a fixed value, how does adjusting the spring, vary the pressure required to move the spring?

What am I missing here?

Thanks in advance.

 

[Stick.]

The spring tension/force depends on two factors, as shown by Hooke's law:

F = k*x

Where F is the spring force, k is the spring constant, and x is the spring's extension/compression from its free state. In this case where you're using the same spring for all cases, k remains constant (as you've already identified). As you turn the adjustment knob, you're increasing or decreasing the compression of the spring (depending on which way you turn the knob), which in turn will change the spring force, and thus make the relief valve harder or easier to open.

 

[RVAmeche]

Note that you can't just crank on ASME relief valves to adjust the set pressure like a regulator or something. The vendor has to come adjust it and update the stamped nameplate.

 

[The Obturator]

thread1203-519334


*** Per ISO-4126, the generic term
'Safety Valve' is used regardless of application or design ***

*** 'Pressure-relief Valve' is the equivalent ASME/API term ***

 

[Jboggs]

"This spring tension depends upon the spring constant, which (by the name itself) is constant."
I think your understanding of the spring constant may be incorrect. The "constant" means that the change in force exerted by a spring is related to the associated change in deflection by a constant factor. It does not mean that the force exerted by the spring is constant. So, the greater the deflection, the greater the force.

 

[btrueblood]

F = kx, but x = xd + xp

xd is the spring deflection (valve opening) in operation,
xp is the preload on the spring obtained by cranking on the adjustment screw.

 

[Channo]

Thank you all for your replies!!!

Guess I mistook spring constant for the force itself.