it is about pressure valves used in hydraulic circuits and its types as well.Its analogy with the electrical circuit.

1. PRESSURE VALVES
VAIBHAV PATHAK
B.TECH 4 YEAR
MECHANICAL
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CONTENTS
1. INTRODUCTION
2. IMPORTANT TERMINOLOGIES
3. SIMPLE PRESSURE-RELIEF VALVE
4. PRESSURE-REDUCING VALVE
5. CONCERNED VEDIO

3. The pressure valve (PV) is a type of valve used to control or limit the pressure in a
system or which can build up for a process upset, instrument or equipment failure,
or fire.
The pressure is relieved by allowing the pressurized fluid to flow from an auxiliary
passage out of the system. The relief valve is designed or set to open at a
predetermined set pressure to protect pressure vessels and other equipment from
being subjected to pressures that exceed their design limits. When the set
pressure is exceeded, the relief valve becomes the "path of least resistance" as the
valve is forced open and a portion of the fluid is diverted through the auxiliary route.
Pressure-relief valves limit the maximum pressure in a hydraulic circuit by
providing an alternate path for fluid flow when the pressure reaches a preset
level
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INTRODUCTION

4. A pressure valve is similar to a fuse in an electrical system. When circuit amperage
stays below the fuse amperage, all is well. When circuit amperage tries to exceed fuse
amperage, the fuse blows and disables the circuit. Both devices protect the system
from excess pressure/current by keeping it below a preset level. The difference is that
when an electrical fuse blows, it must be reset or replaced by maintenance personnel
before the machine cycles again.
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6. IMPORTANT TERMINOLOGIES
Set Pressure : The inlet gauge pressure at which
the pressure relief device is set to open under
service conditions.
Closing Pressure : The value of decreasing inlet
static pressure at which the valve disc reestablishes
contact with the seat or at which lift becomes zero
as determined by seeing, feeling or hearing.
Blowdown : The difference between the set
pressure and the closing pressure of a pressure
relief valve, expressed as a percentage of the set
pressure or in pressure units.

7. Overpressure : The pressure increase over the set pressure of the relieving device.
Overpressure is expressed in pressure units or as a percentage of set pressure.
Overpressure is the same as Accumulation only when the relieving device is set to open
at the maximum allowable working pressure of the vessel.
Coefficient of Discharge : The coefficient of discharge is used for calculating flow
through a pressure relief device.
C.D.T.P. : The pressure at which a pressure safety valve is adjusted to open on the test
stand. The cold differential test pressure includes corrections for the service
conditions of pressure or temperature or both.

8. SIMPLE PRESSURE-RELIEF VALVE
The most widely used type of pressure control
valve is the pressure-relief valve because it is
found in practically every hydraulic system.
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9. It is normally a closed valve whose function is to limit the pressure to a
specified maximum value by diverting pump flow back to the tank. A poppet is
held seated inside the valve by a heavy spring. When the system pressure
reaches a high enough value, the poppet is forced off its seat. This permits
flow through the outlet to the tank as long as this high pressure level is
maintained
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10. PRESSURE-REDUCING VALVE
The second type of valve is a pressure-
reducing valve. This type of valve (which is
normally open) is used to maintain reduced
pressures in specified locations of hydraulic
systems. It is actuated by downstream
pressure and tends to close as this pressure
reaches the valve setting.
A pressure-reducing valve uses a spring-
loaded spool to control the downstream
pressure. If the downstream pressure is below
the valve setting, the fluid flows freely from
the inlet to the outlet.
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11. When the outlet (downstream) pressure increases to the valve setting, the
spool moves to the right to partially block the outlet port. Just enough flow is
passed to the outlet to maintain its preset pressure level. If the valve closes
completely, leakage past the spool causes downstream pressure to build up
above the valve setting. This is prevented from occurring because a
continuous bleed to the tank is permitted via a separate drain line to the tank.
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