We were discussing the working principle and internal construction of pressure relief valve of simple type in our previous
post. Today we will see here the working principle and internal construction of
pilot operated pressure relief valve.
Pilot operated pressure relief valve
Pilot operated pressure relief valve will have one
small poppet type of relief valve and one main relief valve as displayed in
following figure here. Pilot operated pressure relief valve will have two ports
like in simple type of pressure relief valve. One port will be connected with
pump pressure line and other port will be connected with hydraulic reservoir.
Let us see the figure, 3 indicates the spring of
poppet type of relief valve and its spring force setting will be carried out
with the help of pressure adjusting screw. 4 indicates the poppet of poppet
type of relief valve. 2 indicates the spring of main relief valve. Inlet and outlet ports are also displayed in
figure with passage C and D in figure. Passage D indicates here the pilot line.
Main relief valve will have a piston with steam and drilled
passage C as shown in figure. Piston will have same exposed surface area at top
and bottom. Therefore same pressure force will be acting to its both sides, when
pressure at inlet port is less than the setting pressure.
Passage C will be used for keeping the piston in
hydraulically balance condition when pressure at inlet port is less than the
setting pressure. Pressure adjusting
screw will be used for changing the pressure setting of relief valve.
When pressure at inlet port of relief valve increases,
then pressure in passage D will also increase and if pressure at inlet port of
valve exceeds the setting pressure of relief valve then pressure force in
passage D will overcome the spring force and hence poppet will leave its seat.
Small amount of fluid flow starts to flow through
the pilot line to the reservoir and it will create a pressure difference across
the piston and therefore piston as well as steam will leave their seats by
moving upward due to the pressure difference across the piston.
Once piston as well as steam leaves their seats, pilot
operated pressure relief valve will be in open condition and fluid will flow
from pressure line to reservoir directly.
For similar pressure setting and flow, pilot
operated pressure relief valves will be normally in smaller size as compared to
the direct acting pressure relief valves. Pilot operated pressure relief valve
could be operated by remote.
Note:
Pilot operated pressure relief valve is also known
as compound pressure relief valve
Simple pressure relief valve is also known as direct
acting pressure relief valve.
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