Safety Valves Flashcards
The handle on a safety valve is used to
Manually test safety valve
The safety valve on the boiler superheater is set where:
It is the lowest set safety valve
What is the most important valve(s) on a boiler:
Safety valve
Adjustments or repairs to a safety valve can be made by
Manufacturer/authorized representative
What type of valve goes between the safety valve and the boiler
A valve is not permitted between the safety valve and the boiler
When is more than one safety valve required
Boilers with more than 500 〖ft〗^2
There is one safety valve on a boiler. What pressure is it set at
At or below MAWP
Why is the safety valve the most critical valve on a boiler? How are they actuated and then closed
The safety valve is the most important valve because it relieves the boiler of excess pressure that is above the MAWP. They will pop when the pressure exceeds the set point which is usually the MAWP. Once the pressure is relieved below the set point the valve will then close. There are usually two on a boiler and each must be able to relief ½ the boiler capacity if needed
Explain the differences in use for a safety valve and a relief valve
A safety valve is used for gas or vapor services and a relief valve is used primarily for liquid service
Which safety valve should be set to pop first, the one on the boiler or the one on the superheater? Why
The safety valve on the suprerheater should be set to pop first to keep a flow of steam through the super heater so it doesn’t overheat
At what pressures must safety valves close and at what pressure should the safety valve pop open
They should operate with out chattering and be tested to close no more than 4% of the set pressure but not less than 2%. The SV should pop no more than 6% above the highest pressure at which the valve is set or 6% above the MAWP
What is meant by blow-back in relation to safety valves? How can this setting be changed
Blow back is the leakage of a SV after is has popped, which can be caused by wear, dirt or seating of disk
How do you test a safety valve
They are tested annually for both set pressure and closing pressure. This can be done in the field (while boiler is in operation) or bench tested (testing station)
You look at the steam gauge on your boiler and it reads 125psig. The safety valve is set to blow at 100 psig and is not blowing. What do you do? What is wrong
If the manual lever on the safety valve can be operated safely then open the safety valve and allow excess pressure to leave the boiler. Once the pressure drops below the set point then you can close the SV. The opening and closing of the safety valve might have corrected the sticking issue of the SV so pay attention to see if the pressure crepes back over 100psig. If you do this and the pressure doesn’t drop, the pressure gauge might need to be replaced
The safety valve is set to blow at 150 psig. The safety valve lifts and the gauge reads 125 psig. What do you do? What is wrong
It should be freed by operating the lifting lever. If this doesn’t work the valve may have to be repaired or replaced. After repairing or replacing test the safety valve by increasing pressure and noting pop pressure
How do you check a safety valve two ways
Its tested for both set point pressure and closing pressure. This is done annually and bench tested/ adjustment by a qualified rep
Your safety valve opens and will not shut, what do you do
Attempt to stroke valve to disengage from being locked up. Once this is done, bring the pressure back up to have the valve pop and then down to close. If this doesn’t operate in this manner the valve will have to be rebuilt or replaced and tested in place again
Why is more than one safety valve sometimes installed on a boiler
Needing to comply with ASME code section 1, boilers having over 500 ft2 of heating service or exceeding 4000 lb/hr must have two or more safety valves which must be able to relieve half of boiler capacity. Superheaters and reheaters are also required to have a safety valve along with the boiler
How is blow back adjusted on a safety valve
By adjusting the setting or spring on the blow back ring. Once this is done the safety valve should be tested
At what pressure do we test a safety valve
75% of working pressure
How does a relief valve work
Opens gradually with increase in pressure
How often are safety valves tested
Every month
When would you gag a safety valve
Hydrostatic test
Name the 2 types of safety valves
Huddling chamber (most safety valves) Nozzle reaction (used for high pressures)
How much of the total relieving capacity is dedicated to super heaters
Up to 25%
PG 67 safety valve requirements
Each boiler shall have at least one safety valve and if more than 500 sq ft of heating surface or electric boiler with power input more than 1100 KW two or more are required
What is the range of settings for safety valves
One or more at or below MAWP
Highest set safety valve set 3% above the MAWP
The range of settings between the highest set safety valve and the lowest is 10%
On a boiler operating at 1000psi, 1 set at 1000, one set at 1030, and one set at 927psi
How are safety valves mounted when using 2 valves
2 safety valves mounted on a y base, must have roughly equal relieving capacity
2 safety valves mounted individually on a boiler, the smaller valve has to have at least 50% the relieving capacity of the larger valve
What is the difference between a safety valve and a relief valve
A safety valve is an automatic pressure relieving device actuated by the static pressure upstream of the valve and characterized by a full opening pop type action, it is used for gas or vapor service
A relief valve is an automatic pressure relieving device actuated by the static pressure upstream of the valve which opens further with an increase in pressure it is used for liquid service
What is a safety relief valve
It is used for gas/vapor or liquid service. Example (economizers). Acts like a relief valve when there’s water in the economizer and a safety valve when there is steam in the economizer
What is a huddling chamber
It is an increased area of the disc that is exposed to the pressure when the valve starts to open, which overcomes the spring tension causing the valve to pop open
