By Kristian Lund Jepsen, Ramboll Oil & Gas

For control valves allowable seat leakage rates are given by ANSI FCI 70-2 and are not further elaborated

here. The leakage rates listed in the following are the acceptance rates for new valves, i.e. the acceptance levels during the leak testing in the factory. Valves in operation are not covered by the stated leakage rates.

StandardsClassification of allowable leakage rates as well as the test procedures and conditions are stated in several standards, but two of the most often references from oil companies and consulting engineers are:API 598, Valve Inspection and TestingEN 12266-1, Industrial valves - Testing of valves. Part 1: Pressure tests, test procedures and acceptance criteria. Mandatory requirements.These standards covers both strength test (shell pressure test) and seat leakage test.It shall be noted that some valve design standards, such as BS 5351 and

Test description

Valve type

Gate Globe Plug Check Floating ball

Butterfly and trunnion mounted ball

Low-pressure gas Required Optional Required Optional Required Required

High-pressure liq. Optional Required Optional Required Optional Optional

Test description

Valve type

Gate Globe Plug Check Floating ball

Butterfly and trunnion mounted ball

Low-pressure gas Optional Optional Optional Optional Required Optional

High-pressure liq. Required Required Required Required Optional Required

Test description Pressure

Low-preesure gas 4 - 7 barg

High-pressure liq. 110% (*)

Valve size (NPS)

All resilient seated valves

All metal-seated valves except check

Metal-seated check valves

Liquid test(Drops per minute)

Gas test (bubbles per minute)

Liquid test Gas test

≤ 2 0 0 0 (#) (##)

2½ - 6 0 12 24 (#) (##)

8 - 12 0 20 40 (#) (##)

≥ 14 0 (**) (***) (#) (##)

API 6D contain separate leakage rate requirements. API 6D makes reference to ISO 5208, but in practice API 6D valves are often in Europe specified with allowable leakage rates as per EN 12266-1.In the following it is seen that the conditions for tests are quite similar for the two leakage / test standards as the high pressure liquid test will be performed

at 110% of the valve design pressure and the low pressure gas test is carried out at approximately 5 barg pressure.

Seat leakage tests - liquid / gas testsBoth the referred two standards operate with two set test procedures:• A high pressure liquid leak tightness test.• A low pressure gas leak tightness test

Valves: NPS ≤ 4 & ASME class ≤ 1500; NPS > 4 & ASME class ≤ 600

The maximum allowable leakage rates for the leak tests

Valves: NPS ≤ 4 & ASME class > 1500; NPS > 4 & ASME class > 600

Test pressures:Table 3, API 598 test conditions(*) 110% of max. allowable pressure at 38 °C in acc. with the purchase specification.

Table 4:API 598 test acceptance criteria(**): For valves ≥ NPS 14, the max. permissible leakage rate shall be 2 drops per minute per in. NPS(***): For valves ≥ NPS 14, the max. permissible leakage rate shall be 4 bubbles per minute per in. NPS(#): The maximum permissible leakage rate shall be 3 cm3 per minute per in. NPS.(##): The maximum permissible leakage rate shall be 0.042 m3 per minute per in. NPS.

AbbreviationsDN: Nominal Diameter (in mm)ESDV: Emergency Shutdown ValvePN: Nominal pressure (in bar) rating as per EN-standardsNPS: Nominal Pipe Size (nominal diameter in inches)

Table 1: API 598 tests

Table 2: API 598 tests

Seat leakage rates in on/off valves

This article considers allowable seat leakage rates in new gate, ball, plug, globe and check valves. The article shall mainly be seen in relation to valves in the oil & gas industry, but the referred EN 12266-1 is also widely used for industrial valves generally.

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Maximum allowable seat leakage for each leakage rate in cubic millimetres per second:

Test fluid

Rate A Rate B Rate C Rate D Rate E Rate F Rate G

Liquid 0 0,01 x DN 0,03 x DN 0,1 x DN 0,3 x DN 1,0 x DN 2,0 x DN

Gas 0 0,3 x DN 3,0 x DN 30 x DN 300 x DN 3000 x DN 6000 x DN

About the authorMr Kristian Lund Jepsen is Senior Chief Consultant at Ramboll Oil & Gas in Esbjerg, Denmark. He holds an M.Sc. in mechanical engineering from the Technical University of Denmark, Copenhagen, from 1981. He has been with Ramboll Oil & Gas for about 15 years. Mr Jepsen is a technical expert within the oil and gas production and processing areas, with special focus on mechanical equipment and systems including valves. Earlier in his career, he has held positions in large energy companies such as Maersk Oil and Halliburton.

API 598. Leakage ratesThe tests are carried out as per Tables 1 to 4 on previous page. It is seen that in many cases the options possible must be specified on a case by case basis by the specifier.

EN 12266-1, Leakage ratesThe selection of, if the test is carried out as low pressure gas test or high pressure liquid test is decided by the valve manufacturer.

Test description

Pressure

Low-preesure gas 5 - 7 barg(**)

High-pressure liq. 110% (*)

Test pressures:

Table 5: EN 12266-1 test conditions.(*) 110% of max. allowable differential pressure at room temperature.(**) Applicable for valvessizes up to DN 80 for all pressure ratingssizes > DN 80 - DN 200 for pressure ratings up to PN40 / class 300.

The rate A is here normally the class specified for soft seated valves, especially those for ESD use including riser ESDV’s.

ConclusionFor riser valves and emergency shutdown valves with metal-to-metal seats a rate D would often be specified in the oil/gas industry. However, for riser ESDV’s, leakage rates B or C are also in some cases specified. In general there seems to be no correlation between the allowable leakage rates specified for new valves and that of

the valves in service, i.e. after they have been in use for some time. Therefore it is appropriate to apply a requirement not stronger than the rate D for new valves. This is also in line with the recommended leakage rates in ISO 14313.

It shall be mentioned that leakage rates of safety critical valves in service cannot be evaluated against the above mentioned leakage rates. An approach including risk analysis on a case-by-case basis must be applied for valves in service.

Fig. 1: Offshore metal-to-metal seated, trunnion type riser valve (24” NPS) to API 598. Leakage rate EN 12266-1, rate D.

Fig. 2: 1” NPS class 150, soft seated floating ball valve. This valve will require a leakage rate A according to EN 12266-1.

Fig. 3: 8” NPS, class 150 wafer check valve, metal-

to-metal seated. Check valves are normally not

considered to be tight and a leakage rate E, F or G

would be sufficient for most check valves.

Table 6: EN 12266-1 test acceptance criteria.

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