Safety Valve Floating in Thermal Power Plant Plants :

Safety Valve : Safety Valve is a one type of valve that automatically actuates when the pressure of inlet side of the valve increases to a predetermined pressure, 

to open the valve disc and discharge the fluid ( steam or gas ) ; and when the pressure decreases to the prescribed value, to close the valve disc again.

Purpose of Safety Valve : Safety valve are used to prevent the boiler pressure parts from being pressurized more than the design value  

Types of  Safety Valve :

Assisted Type  & Non-Assisted Type Safety Valves :

Assisted Type Safety Valves : Countries Like Germany They are Used these types of safety valves , the Pressure Raise is sensed by a Pressure Sensor and 

the safety valve is opened to release the excess pressure.

Non-Assisted Type : It is Nothing but a Spring loaded  safety valve , Countries like America and India are using these type of safety valves,

When the steam pressure exceeds the normal working pressure , the valves  rise up against the action of the spring and allow the steam to escape 

from the boiler till the pressure in the boiler reaches its working pressure.

Parts of a Safety Valve:

Location of Safety Valves :

Sample of Safety Valve Locations in our Plant.

Location	Sl. No.	Type of Safety Valve	Valve set Pr. M Pa	% of Blow Down	Relieving Capacity TPH
BOILER DRUM	1	HE-96W	20.27	4	286.662
	2	HE-96W	20.68	4	295.752
	3	HE-96W	20.88	4	300.289
SUPER HEATER OUTLET	1	HCI-88W	19.06	3	119.361
	2	HCI-88W	19.09	3	119.627
REHEATER INLET	1	HCI-46W	4.85	4	167.344
	2	HCI-46W	4.90	4	169.223
	3	HCI-46W	4.95	4	223.484
	4	HCI-46W	5.0	4	225.697
REHEATER OUTLET	1	HCI-48W	4.56	4	76.087
	2	HCI-48W	4.76	4	79.459
Super Heater Outlet of PCV Valve	1	ZOL730-ABD5P1	18.87	2	220.0
Reheater outlet of PCV valve	1	Z7L7-34CBRE5p1	4.51	2	115.0

Points to Remember :

Ø  Set Pressure

Ø  Reset Pressure

Ø  Relieving Capacity

Ø  % of Blowdown 

Set Pressure: The Pressure at Which the safety valve pop-up is known as set pressure.

Reset Pressure: The Pressure at which the safety valve seated fully.

Relieving Capacity : When the safety Valve Pop-Ups the quantity of steam released to atmosphere is known as Relieving capacity, these relieving capacity will vary because due to flow rate.

For Eg : Drum Safety Valve has more relieving capacity than MS Line safety Valve relieving capacity.

% of Blowdown : Blowdown is the Difference the Set Pressure and the Reset Pressure, Once the valve pops or opens, the pressure in the system may still raise. Because of this, the safety valves are designed to go full open within an overpressure level of 3%.The valve will have to be set in such a way that the reseat pressure is 95% of the set pressure. The percentage of this is called the "percentage blowdown" and is limited to a maximum of 5% to avoid wastage of steam produced. However the spring loaded safety valves cannot be set for a blow down lower than 3% as this will make the valve chatter.it may leads to affect the Disc and Spring Tension.Values of Blowdown are set by codes (ASME,API,NFPA) and their typical range is 7% to 10%. 

Safety Valve Floating Procedure :

Normally the highest set pressure valve is the valve floated first. While setting this valve other safety valve is gagged, Raise the Boiler Pressure Nearer to the Safety Valve Set Pressure and modulate the firing,

Safety Valve Floating Can be done in two ways , By Increasing the Boiler Pressure nearer to the Safety Valve Set Pressure by throttling the Boiler Pressure by Start-up-Vent, Here the exact Set Pressure and Reset Pressure will be Identified,  After Safety Valve Pop-up Adjust the Boiler Pressure by Start-up vent.

Then Raise the Boiler Pressure upto 80% of the Rated Pressure and Then Manually Raise the Pressure by Hydraulic or Pneumatic Jack, in This Method we can able to identify the Set Pressure but not the Reset Pressure and even Blowdown % also.

Safety Valve Pressure Setting:

There are Two Things to be Remember , They are Adjusting Bolt and Lock Nut, By Tightening the Adjusting Bolt we can Increase or Decrease the Set Point of Safety Valve, the Lock nut is for avoid Loosening of Adjusting Bolt. If the Pop-up Pressure is Low Tighten the Adjusting Bolt and Vice Versa.

Any Adjustments in safety Valve should be done only below 10% of the actual Popping Pressure.

Blowdown Adjustment :

Raising the Blowdown Popping Pressure Increase the blowdown range and reduce the popping pressure reduce the blowdown range. These Blowdown range can be adjusted by ring adjustment.

There are Two Rings available in Safety Valve one is the Guide Ring ( Blowdown Adjusting Ring ) and another one is the Nozzle Ring

The Guide Ring can be Change Either Right or Left , Turning the guide  ring to the right raises it and reduces the blow down. Turning the guide ring to the left lowers it and increases the blow down. Only Notch Adjustments to be done. The nozzle ring adjustments are very rare because its carefully set by the manufacture before its leaves the manufacturing area, The Nozzle ring adjustment done when safety valve shimmering happens , the nozzle ring adjustment are very effective in reducing the blowdown for high pressure safety valves.

Video for Safety Valve Floating : Click Here

Safety Valve Floating Images :

Person Adjusting the Adjusting Bolt for Increasing Pressure.