Question 1

can this relief valve be used to drain water from the air tank

Accepted Answer

Its' purpose is to relieve pressure - that's what it's designed for.I would not use it to drain water from a tank; it would likely corrode the components and compromise its' effectiveness.You will never find one of these on a compressor at the bottom of the tank - there's a drain there; it's always placed toward the top away from water build up.

Question 2

Does it release right at 200psi because I have a 200psi pressure switch.

Accepted Answer

If working properly the safety will blow at 200, if the safety valve psi is lower than the pressure switch set point it will blow before it reaches the set point, so the comp will never shut off, the safety valve should be about 30 lbs more than the pressure switch set point.
A 200 psi safety switch is common on a comp set to 165-175 psi.

Question 3

What is the maximum negative temp that this valve will work with? For example -20c?

Accepted Answer

I would not trust a $5 relief value without checking with the manufacture first.

Question 4

if you want to reduce pressure mechanically through the relief valve , do you have to unthread the valve or is there a special action for that?

Accepted Answer

No, you don't unthread the valve body. You just have to pull on the key ring. A spring keeps the valve closed; at the pressure the valve is rated for, the air pressure overcomes the spring force and allows the valve to open and relieve pressure. Pulling on the key ring hard enough allows you to overcome the spring force and open the valve; when you release the key ring, the spring should immediately close the valve.

Manufacturer	Control Devices
Part number	ST25-1A200
Item Weight	45 g
Product Dimensions	12.7 x 2.54 x 2.54 cm; 45 g
Item model number	ST25-1A200
Size	0.25 Inch
Colour	Original Version
Style	Threaded
Material	Brass
Item Package Quantity	1
Flow rate	178 Cubic Feet Per Minute
Measurement System	Inch
Specification met	certification
Included components	Safety Valve
Batteries Required?	No

Control Devices - ST25-1A200 ST Series Brass ASME Safety Valve, 200 psi Set Pressure, 1/4" Male NPT

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