RELIEF & SAFETY VALVES
FOR THERMAL POWER PLANTS
Pressure relieving valves
(relief, safety and safety
relief valves) are used
throughout the thermal
industry to provide
of pressurized systems
They are designed to
mitigate pressure rise in
the system to below a
defined design value.
2.TYPES OF PRESSURE RELEIVING
An automatic pressure
relieving device actuated
by the pressure
upstream of the valve
and characterized by full
opening pop action .
It is used for steam ,gas
or vapor service
Safety valves may be
spring loaded or pilot
Safety relief valves are pressure relieving
devices actuated by the inlet static pressure
and characterized by rapid opening or
“popping” action, or by opening in proportion
to the increase in pressure over the opening
pressure, depending on application.
Safety relief valves can be used for either
liquid or compressible fluid service.
The primary difference between a safety relief
valve and a safety valve is that the safety relief
valve has a fluid tight bonnet, allowing it to be
used for liquid service.
5.POWER OPERATED RELIEF VALVE
Power operated relief valves (PORVs) are
pressure relieving devices which require an
external power supply for actuation.
These valves are typically controlled by an
electrical signal resulting from high system
pressure or manually from the control room.
The electrical signal initiates the relief action
by activating the valve actuator, either
electrically (ERV) or pneumatically.
6.REQUIREMENT OF SAFETY VALVE
( ASME SEC 1)
Each boiler shall
have at least one safety
valve or safety relief
valve and if it has more
than 47 m2 of bare
surface, or if an electric
boiler has a power input
more than 1,100 kW, it
shall have two or more
safety valves or safety
When only two valves are used,
capacity of smaller shall not be less
than 50% of the larger. (PG-71.1)
8.SETTING OF SAFETY VALVE FOR BOILER
The safety valve or safety relief valve
capacity for each boiler shall be such
that the safety valve, or valves will
discharge all the steam that can be
generated by the boiler without allowing
the pressure to rise more than 6%
above the highest pressure at which any
valve is set and in no case to more than
6% above the maximum allowable
9.CAPACITY OF VALVE
The capacity of the safety valve is the
quantity of steam it can relieve when
the valve is fully open.
The safety valves in a boiler are so
selected that the capacity of spring
loaded safety valves on drum and
superheater put together will be more
than the 100% steam generation of the
10.Super heater valve
At least one valve shall be installed
on the superheater outlet. it is good
practice to size the valve to relieve a
approximately 20% of the total boiler
capacity to protect the tubes against
Drum valves must relieve a
minimum of 75% of total steam
generation of boiler.
11. Reheater safety valve
Every reheater shall have one or more
safety valves, such that the total
relieving capacity is at least equal to the
maximum steam flow for which the
reheater is designed.
Boilers having reheaters must have at
least one safety valve on reheater outlet
capable of relieving a minimum of 15%
of the flow through the reheater.
12.Lever safety valve
Deadweight or weighted lever safety
valves or safety relief valves shall not
be used . (ASME SEC 1)
13.SAFTEY VALVE CONNECTION
The safety valve connection to the
Boiler shall be independent of any
other connection and without any
unnecessary intervening pipe or
The opening or connection between
the Boiler and the safety valve must
not be smaller than the valve inlet.
15.SAFETY VALVE DISCHAGRE DRAIN
The discharge drain outlet must be piped
full size without any shut-off valves,
independent of other piping.
Install piping with sufficient flexibility to
allow for free expansion and properly
support so there is no strain on the safety
Pipe to a safe point of discharge to prevent
any possibility of personal injury and within
18" from the floor .
16.Sequential lift series
Sequential lift series applies when there
is more than one pressure relieving
device in the system.
17.1.Capacity of Safety & Relief
Valves for Supercritical Boilers
Spring loaded safety valves:
Separator and superheater- combine capacity
105% BMCR( minimum)
Reheater – combine capacity 105% of reheater
flow BMCR (minimum)
Electromatic relief valves (ERV):
Superheater- 15% BMCR
Reheater -15% BMCR
17.2.Safety valves in supercritical
boiler -660 MW power plant
s.n description Spring
2 SH outlet 4 4
3 CRH 6
4 HRH 2 4
18.SAFETY VALVE SETTING
-500 MW BOILER
The valves are selected for following
Maximum evaporation of boiler is 1675000
kg / hr
Maximum allowable boiler drum press is 207
kg/cm2 (g) and operating press is 194 kg/cm
SH steam operating temperature is 540 c deg.
Maximum flow through reheater is 1445530
18.1.Total % of Evaporation of
safety valve-500 MW boiler
S.N LOCATION No of safety
TOTAL % OF
1 BOILER DRUM
SH - 2
2 SH SH - 5 30.75
3 CRH+HRH CRH- 4
HRH - 4
4 HRH HRH - 4 33.34
18.2.Relieving capacity of drum &
SH safety valves -500mw
relieving capacity from drum safety valves
is 86.76% of total evaporation of boiler.
relieving capacity from superheater safety
valves is 22.38% of total evaporation of
Total relieving capacity of drum and SH
safety valves -109.14% =(86.76+22.38)
of total evaporation of boiler.
18.3.SAFETY VALVE STTING-500 MW BOILER
DRUM & SH
Every safety valve used on a super heater
or reheater discharging superheated steam
at a temperature over 230 deg c ,shall have
a casing ,including the base ,body ,and
bonnet and spindle ,of steel alloy or
equivalent heat resisting material.
Materials used in body to bonnet or body to
yoke bolting shall be in ASME B 16.34.
Cast iron seats and disks are not permitted.
Blowdown is the difference between the
set pressure (“popping” pressure) and the
resetting pressure of a pressure relieving
valve. This pressure is commonly expressed
as a percentage of the set pressure such as
Another way of describing blowdown is to
say that it is the difference between set
pressure of the valve and system pressure
when the valve recloses.
21.1.Safety valve for tanks and pumps
Low-pressure storage tanks must be protected
when liquid is pumped into or out of the tank.
This is required to prevent overpressurizing or
collapsing the tank when liquid is being moved
from or to the tank.
Positive displacement pumps and reciprocating
compressors should have pressure relief valves
on their discharges to relieve the fluid if the
discharge should be blocked.
21.2.Safety valves for heat exchangers
Heat exchangers that have valves on both
the inlet and outlet can be isolated if both
valves are shut.
Safety/relief valves should be provided to
protect the heat exchanger from the effects
of thermal expansion of the liquids that
may be isolated within the heat exchanger.
Consideration should also be given to
protection of equipment on the low
pressure side if a tube within the heat
exchanger should rupture.
21.3.SAFETY VALVES AT TURBINE CYCLE
Aux steam after desuperheater.
LP heaters (shell side).
HP heaters (shell side).
Before BFP Turbine inlet (steam).
If a muffler (silencer) is used on a safety
valve or safety relief valve; it shall have
sufficient outlet area to prevent back
pressure from interfering with the proper
operation and discharge capacity of the
Mufflers shall not be used on high-temperature
water boiler safety relief
Silencers on all lowest set pressure safety
24.Feedwater supply and safety
Boilers having more than 500 ft2
(47m2) of water heating surface shall have
at least two means of feeding water.
Each source of feeding shall be capable of
supplying water to the boiler at a pressure
of 3% higher than the highest setting of
any safety valve on the boiler.
1. ASME- Boiler and Pressure Vessel
Code Section I, Power Boilers, and
Section VIII, Pressure Vessels.
2. ASME- Performance Test Code
PTC-25, Safety and Relief Valves.
3.ASME/ANSI power piping B31.1
26.Layers Of Protection-
Safety valves are
Part of mitigation
System in a power
plant as per
Typical risk reduction
methods found in a
Thermal power plants