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:
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?
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