Friday, 1 September 2006

Control valve cryogenic test procedure

I was asked to create this control valve cryogenic test procedure for one of the world’s leading engineering companies.

1 Introduction


2 Location of tests in manufacturing programme

3 Test temperature of minus (-)196 Celsius

4 Test temperature above minus (-)196 Celsius

5 Equipment specifications

1 Introduction


The company has the facility to carry out cryogenic testing of control valves at temperatures as low as minus (-)196 Celsius.  Tests conducted at these temperatures include:

(a) Seat leakage test
(b) Gland leakage test
(c) Body/bonnet joint leakage test
(d) Mechanical operation test

The tests are carried out on the valve when the required test temperature has been achieved and maintained for one hour.  This allows for a uniform temperature distribution throughout the assembly.  The allowable deviation from the desired temperature is ±3 Celsius degrees over this period.

There are two modes of operation for the cryogenic test plant, depending upon the required test temperature.  Heat transfer is effected by immersing the test valve in a bath of liquid nitrogen at its saturation temperature for a test temperature of minus (-)196 Celsius.If the required test temperature lies above minus (-)196 Celsius then an evaporator is used and this creates heat transfer by passing a mixture of evaporated nitrogen and cold air around the test valve.

2 Location of tests in manufacturing programme

All tests are carried out on the assembled valve and actuator upon completion of the hydrostatic and seat leakage tests.  Prior to testing, the valve is fully disassembled and degreased, ready for cryogenic testing.  The valve internal and external surfaces of the valve are to be free of moisture, dirt and metal particles.  The valve is fully disassembled, to check for any permanent distortion of components, upon completion of the test.

3 Test temperature of minus (-)196 Celsius

Blank flanges, with ¼” NPT connections, are bolted to the valve body and compress a PTFE gasket to provide an efficient seal.  Alternatively, for valves with butt welded ends, blind hubs are used.

The 1/4" metal tubing from the helium cylinders is attached to the inlet side of the valve. The tubing has a series of coils to increase the transfer of heat from the incoming helium, thereby reducing the temperature difference between the helium gas and the valve.  At the outlet of the valve, 1/4" metal tubing is connected to a bubble meter and flow meter.  Calibrated thermocouples are attached to the valve in five places :

(a) inlet blank flange (side)
(b) inlet blank flange (top)
(c) body/bonnet joint (body flange)
(d) body stud/nut
(e) actuator locking ring

The valve is carefully lowered into the insulated cooling tank, prior to the admission of liquid nitrogen.  A sufficient clearance must exist between the valve and the tank walls to allow for good circulation of the coolant.

Proof tests of the valve are carried out at ambient temperature to determine whether the test should continue and these tests are identical to those to be done at the test temperature.  The results of these tests should be better or equal to those required at the test temperature.

Upon satisfactory completion of the initial proving test, the valve is now ready to be cooled down to (-)196 Celsius.  Liquid nitrogen is poured into the insulated tank up to the body/bonnet joint.  Initially, rapid vaporisation of the nitrogen takes place, making it necessary to frequently pour liquid nitrogen into the vessel.  As the valve temperature is reduced to near the test temperature, only occasional 'topping up' is required to maintain the liquid nitrogen slightly above the body/bonnet joint.

When the test temperature has been achieved (± 3 Celsius degrees), a period of one hour must elapse to allow for the valve internals to reach the required temperature.  A purge of helium gas is required to be passed through the partially open valve to prevent the ingress of moisture throughout the cooling down operation.

The position of the valve is chosen so that the packing box, located in the valve bonnet, is clear of the cold boil-off gas.

(A) Seat leakage test - With the valve in the fully closed position, the helium gas pressure is raised to the seat test pressure of the valve.  The seat test pressure is equal to the maximum shut-offpressure in service.  This pressure may not exceed the cold pressure rating of the valve.  The valve trim leakage (if any) is recorded over a ten minute period and must not exceed a previously agreed value between the client and the company. Typical values for single seated globe valves, expressed as a percentage of the maximum flow coefficient (Cv), may be:

(a) 0.1000% metal piston rings
(b) 0.0060% nitrile '0' rings, balanced design
(c) 0.0050% p.t.f.e. lipseals
(d) 0.0020% metal/metal (standard)
(e) 0.0002% metal/metal (special lapped), solid design
(f) 0.0001% soft face
(g) bubble tight (special)

(B) Gland leakage test - With the body pressurised to the seat leakage test pressure, a 50% (by volume) solution of teepol/alcohol is brushed on to the top of the packing box.  Gland leakage, detected by the formation of bubbles, is not allowed. It may be necessary to slightly adjust the packing box flange nuts to achieve zero leakage because of component contraction during 'cool-down'.

(C) Body/bonnet joint leakage test - With the body pressurised to the seat leakage test pressure, careful examination of the body/bonnet joint area is undertaken.  The body/bonnet joint is submerged below the surface of the liquid nitrogen and any escape of helium gas (evidenced by bubbles) is not acceptable.

(D) Mechanical operation test - With no pressure in the valve body, i.e. just sufficient gas to prevent the ingress of moisture, the valve is stroked twenty times.  The valve should travel over its entire stroke length ina smooth action, with no jerks or jumpy action.

4 Test temperature of above (-)196 Celsius

The test valve is firmly supported on a platform above the evaporator.  Initial proving tests are carried out at ambient temperature to determine whether to proceed with the cryogenic test.

Upon satisfactory completion of the proving test, the evaporator is filled with liquid nitrogen.  A multi vane centrifugal fan draws air from the atmosphere and forces it through a heat exchanger.  This creates a reduction in the temperature of the air and evaporation of the liquid nitrogen.  The mixture of air and nitrogen then flows past the test valve in such a manner as to allow heat from the test valve to warm the mixture again, thereby reducing the temperature of the test valve.  The temperature of the test valve is regulated by the action of the centrifugal fan.

Various tests, as described in Section 3, are then carried out on the valve at the test temperature.

5 Equipment specifications

Fan
Type : 9” diameter Halifax multi-vane centrifugal fan
Outlet velocity : 4880 feet per minute
Speed : 2300 revolutions per minute
Static pressure : 1.75” water gauge

Heat exchanger
Tube diameter : 6” nominal
Tube length : 29 feet immersed
Evaporator Volume : 50 cubic feet

Insulated tank
Inside width : 1 foot
Inside length : 3 feet
Inside height : 2 feet

Overhead crane facility
Safe working load : 5 tonnes

Insulated enclosure
Insulation : 6" thick expanded polystyrene block
Housing : softwood
Inside width : 6 feet
Inside length : 6 feet
Inside height : 6 feet

Insulated pit
Insulation : 6" thick expanded polystyrene block
Sides : Concrete
Inside width : 6 feet
Inside length : 6 feet
Inside height : 6 feet

Electrical equipment
Five thermocouples
Digital voltmeter

Flow equipment
Alcohol Bubble meter
Positive displacement meter

1 comment:

sriram nair said...

Thanks for this useful post. but i need some clarification. Attaching the thermocouples ON the valve body, the thermocouples are exposed to the Liq.nitrogen. does it make any difference on the reading we get which we can assume to the actual temperature the valve attains? do we need to have a probe inside the valve?