Online Safety Valve Testing 2

[jaffaotc]

Experts,

I would like to know about the online testing of the safety valves...... i was wounder when our one client told me that its not recommended to online service testing.

Since boiler safety valves are almost welded, if is that case how can we test those valves...........?

Jaff

 

[ash9144]

I guess by "safety valve" you mean relief valve. RVs should not be welded in place.
What do you mean by "online testing"?

 

[JLSeagull]

The term "safety valve" applies to the open body relief valves for steam applications. Some steam valves are for welded inlets.

Perhaps online testing could apply to pilot operated valves with the valve body remaining in-place and testing the pilot setting. Perhaps other valves could be tested in-place by blocking the inlet and applying pressure via a connection between the block valve and the relief valve. Many steam valves such as ASME Section I would not have an inlet block valve or test connection port.

 

[gerhardl]

As english is not my native language I always try to make sure what 'exact device and rules' we are talking about. The answer and exact meaning could vary from country to country.

As implicated from answers above:

a) Some regulations will sometimes require that safety valves are taken out from the line and tested at intervals in test benches and re-certified Approved for leakage and setting point.

b) Under certain conditions and for certain constructions valves can be tested to a certain degree inline. Simplest for function/movement for simpler types with lifting handle for disc, to more complicated tests if a revision valve before safety valve can be closed and a side entry opening (closed with valve) before the valve and after revision valve are provided for different test purposes.

c) More seldom the complete piping systems with safety devices can be tested to relief pressure (if relief pressure is within acceptable -normal operational pressure range - for the complete installment with all pipeline components) and if relief devices can bear repeted release operations.

The general advice is always to check the exact local requirements against existing regulations.

 

[vpl]

jaffaotc

Try googling "Trevi testing". Not saying it's the only company, but it was the one that did the in-place testing of some main steam safety valves at a plant where I worked.

Patricia Lougheed

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[jaffaotc]

Patricia,

i want to know more about the online testing of relief valves, basically what are the tools or equipment used for testing.

jaffaotc

 

[JAlton]

vpl has put you on the right track. The Trevitest Device is one brand of Lift Assist Device (others include KISS, EVT, PVT AVK, Hydroset, SPIVOT). SOme are manufactured by Pressure Relief Valve Companies, others by Valve Repair Shops and some are third party vendors with PRV background. A Pressure Relief Valve typically uses a spring (Force S) to hold a movable sealing member (disc) closed on a fixed sealing member (nozzle) until the system pressure (Force P) overcomes the Spring and opens the PRV to relieve an overpressure situation. The PRV works on the simple principle that IF Force S > Force P THEN the PRV remains closed (Static Force Balance Principle). However, IF Force P > Force S THEN the PRV is Open.

The problem is that most Steam Power Generating Stations, whether Fossil or Nuclear, are such high pressure and under so much demand for electricity that they do want to take the time or liability to overpressure their boilers to test Safety Valve Set Pressure. Use of the Lift Assist Device is permitted by ASME Code (both Sec. I, Power Boilers, & Sec. VIII, Pressure Vessels) to determine the Set Pressure of a PRV (Safety, Safety-Relief or Relief Valve).

The Lift Assist Device (LAD) applies a third (or auxiliary) force (Force L), typically Hydraulic, but pneumatic devices exist, to the Spindle or Stem of the PRV. The formula then becomes L + P = S. Lift Assist plus System Pressure = Spring Force. IN order to accurately determine Set Pressure several things are necessary. Accurate measure of System Pressure is required. PRV Effective Seat Area is required and Area of the Lift Assist Hydraulic Ram is required. The relationship of the PRV Seat Area to the Hydraulic Ram Area is calculated and the value of the Force applied to the Ram is used to calculate the equivalent force on the PRV Seat Area. The Lift Assist force required to pull the PRV Disc away from the Nozzle (effectively opening the PRV) is then added to the system pressure to determine the PRV Set Pressure.

The Equipment Required is a Calibrated System Pressure Gauge, a Lift Assist Device with an adapter to connect to the PRV Stem, a Calibrated Load Cell to measure the Force Applied to the PRV Stem, a calculator or PC with a program to determine the Set Pressure from the input data and most importantly, a Valve Technician who knows PRV Terminology, and is familiar with the Test Equipment.

I hope this information is helpful. You may also want to read thread 1203-180240 which relates to this topic.

JAC