2 - TCA spa

Figure 1. 1DR30 Direct acting poppetPressure (1)Tank (2)Figure 1 shows a section through a typical relief valve wherethe poppet design allows for a relatively low pressure rise dueto increase in flow. A problem with this type of valve is that toomuch flow can cause the valve to have a negative pressurerise causing the valve to go unstable with fluctuatingpressure.The re-seat and repeatability of the valve depends upon thehysterisis. Internal seals cause friction against the bore as thevalve tries to close. If a seal is under pressure then thehysterisis increases, graph 2.Graph 2. Relief Curve showing the effect of hysterisis1DR30-10S SET @ 100Bar CRACK PRESSUREFLOW = 30L/MINP/BAR150120906030PRESSURE VS FLOW05.0 15.0 25.0 35.0 45.0Caner3 6.7.2001 14:03LPMFigure 2 shows a typical pilot operated, spool type relief valvethat gives good control over varying flows. This valve, due toits design, allows a high flow to pass with very little rise ininlet pressure. The valve has a good re-seat and goodrepeatability due to there being no internal seals. A pilotoperated relief valve is suitable as a main pressure control butdue to the two stage design it is not suitable for safetyapplications where speed of operation is important. In thecase of a rapid increase in inlet pressure the system will besubject to a longer pressure spike than if a direct acting valvewhere used.Figure 2. 1AR100 pilot operated spoolPressure (1)Tank (2)Increasing the flow capacity of a direct acting valve byreducing the area over which the pressure acts is possible,figure 3 shows a differential area poppet type relief valve thathas the capability of very fast action and a high flow capacityfor its size. The internal seal is subject to inlet pressure so thevalve will display relatively poor re-seat characteristics.The design of the poppet is such that as the valve begins toopen the flow past the poppet draws oil from the springchamber (by venturi effect over the small holes in the poppetannulus) causing initial over opening. This removes most ofthe pressure spike. The valve is therefore highly suitable asprotection for actuators.Figure 3. 1LR100 direct acting differential areaPressure (2)Tank (1)A poppet valve should not leak more than 1/3 cc/min up to thecracking pressure allowing it to be placed in a line where lowleakage is important, and performing duties such as a serviceline relief.A simple relief valve like this will give cost effective reliefprotection to small systems or where the valve is not the mainpressure control but a pressure limiting device. They are notgenerally suitable for high flows because the spring wouldhave to be of excessively high rate which would give anunacceptably steep relief curve.