Such valves are intended to open when the pressure in their fluid inlet tubing exceeds a predetermined value, known as the set pressure. The fluid under pressure is generally air or steam.
The operations of setting the pressure of a safety valve and of checking the value of the set pressure are not generally performed directly on the installation which uses the safety valve, but rather on a test bench.
A first difficulty stems from the setting conditions on a bench not being the same as the real operating conditions, for example temperature and the back pressure downstream from the valve. This requires corrections to be made relative to the setting as made on the bench.
It has been observed that a valve releases, ie. its valve plate lifts, in a manner which is neither complete nor immediate. Accompanying FIG. 1 plots the displacement x of the valve plate as a function of pressure P for a valve operating in an industrial installation. The full line is the theoretical curve while the dashed line is the real curve.
The portion ABCD corresponds to the valve releasing under increasing pressure: for a given pressure P.sub.do referred to as the "beginning of release pressure", a sharp valve plate lift is observed (line AB) corresponding to the plate losing contact with its seat and to equilibrium being established between the force exerted by the pressure at the beginning of release and the forces applied by the setting spring. To fully open the valve (point D) the pressure must be increased to a value P.sub.o referred to as the "opening" pressure.
On closure (portion DEFGH of the curve) with falling pressure, the pressure must drop to a value P.sub.f referred to as the closure pressure.
The significant parameter of the valve's operation which needs to be measured accurately is the beginning-of-release pressure, ie. the pressure at which the plate lifts significantly.
A first currently proposed method of bench setting consists in progressively increasing the air pressure upstream from the valve and in waiting for the bang which marks the beginning of release: the corresponding pressure is then noted. However, this method has the drawback of requiring the flow geometry upstream from the valve to be modified to take account of the differences compared with real operating conditions. In particular, the valve plate must be enabled to lift even under small fluid flow (the flow available to a bench installation). This effect is obtained by modifying the flow geometry upstream from the valve, eg. by changing the position of a ring screwed onto a part called the "nozzle" and forming the valve seat. Maximum lifting force is thus obtained under conditions which are practically static (very low flow rate).
Further, the pressure must rise with a steep enough gradient to compensate for losses just before the beginning of release when air begins to stream out.
In a second method which is also currently proposed, and which does not require the flow geometry of the valve to be modified, the fluid is water, and valve opening is detected visually. However, there is a risk of confusion betwen the range over which the valve is no longer fully sealed (the valve starts sweating) and the genuine beginning of valve release. Visual examination is less accurate in this respect than hearing the bang of the previous method. Further, since the pressure rises with a gradient which is ill defined, there is a danger of bouncing which will damage the valve.
Preferred embodiments of the present invention remedy the drawbacks of both of these known methods by recreating on the bench the real conditions of instantaneous flow rate found in use, in conjunction with a steep pressure gradient.