Vana Sızdırmazlık Sınıfları

Types of Leakage

There are two types of leakage from a valve,

• External Leakage: Fugitive emissions from the valve to atmosphere
• Internal Leakage: Leakage through the valve but contained within the piping system

Reasons of the  Leakage

First of all , the right valve type and materials should be selected by considering the process and applciation details . Note that most of the leakage problems occur because of wrong selection of valve type or materials.

Common causes of leakage to atmosphere:

Gasket between valve-body and valve-bonnet is damaged

Common causes of leakage through the valve

Valve is not fully closed

       This can be due to various reasons, including;

       Valve seat is prevented from closing fully due to dirt, rust, or line debris

Insufficient actuator travel

The seat is damaged

The seal is damaged

American Petroleum Institute (API)

The American Petroleum Institute API598: Valve Inspection and Testing,

covers the testing and inspection requirements for gate, globe, check, ball, plug & butterfly valves.

It has acceptable leakage rates for liquid as well as gas testing.

All valves built to the various API standards are required to meet API-598 leakage criteria  prior to shipment .

API 598 states for shell and backseat tests, no visible leakage is permitted.

If the fluid is a liquid, there shall be no visible evidence of drops or wetting of the external surfaces

(no visible leakage through the body, body liner, if any, and body-to-bonnet joint and no structural damage).

The allowable rate for leakage of test fluid past the seats, for the duration of the tests, is listed in the following table:

1. There shall be no leakage for the minimum specified test duration. For liquid test, 0 drop means no visible leakage per minimum specified test duration. For gas test, 0 bubble means less then 1 bubble per minimum specified test duration.
2. The maximum permissible leakage rate shall be 0.18 cubic inch (3 cubic centimeters) per minute per inch of nominal pipe size.
3. The maximum permissible leakage rates shall be 1.5 standard cubic feet (0.042 cubic meter) of gas per hour per inch of nominal pipe size.

Manufacturer’s Standardisation Society (MSS)

The MSS standard MSS-SP-61: Pressure Testing of Valves was originally adopted in 1961. It was developed for the purpose of providing a uniform means of testing valves commonly used in the “full open” and “full closed” type of service.

It is not intended for use with control valves. Refer to standard ANSI/FCI 70-2 for Control Valves.

Pressure Testing of Steel Valves, relates to seat closure tests, and defines the following leakage rates:

Gate, Globe, Ball valves

10 cc/hr per inch of nominal pipe diameter

(Example: A 6″ globe valve is allowed to leak 60 cc/hr in a test)

Check valves

40 cc/hr per inch of nominal pipe diameter

All shutoff or isolation valves specified to MSS-SP-61 must pass the above standards.

American National Standards Institute (ANSI)

The ANSI standard FCI 70-2: Control Valve Seat Leakage,

establishes a series of six seat leakage classes for control valves and defines the test procedures.

Class I. Is also know as dust tight and can refer to metal or resilient seated valves,

no actual shop test is made.

Class II. This class establishes the maximum permissible leakage generally associated with commercial double-seat control valves or balanced single-seat control valves with a piston ring seal and metal-to-metal seats.

0.5% leakage of full open valve capacity

Service dP or 50 psid (3.4 bar differential), whichever is lower at 50 to 125 oF

Test medium air at 45 to 60 psig is the test fluid

Class III. This class establishes the maximum permissible leakage generally associated with Class II (4.2.2),

but with a higher degree of seat and seal tightness.

0.1% leakage of full open valve capacity

Service dP or 50 psid (3.4 bar differential), whichever is lower at 50 to 125 oF

Test medium air at 45 to 60 psig is the test fluid

Class IV. This class establishes the maximum permissible leakage generally associated with commercial unbalanced single-seat control valves and balanced single-seat control valves with extra tight piston rings or other sealing means and metal-to-metal seats.

0.01% leakage of full open valve capacity

Service dP or 50 psid (3.4 bar differential), whichever is lower at 50 to 125 oF

Test medium air at 45 to 60 psig is the test fluid

Class V. This class is usually specified for critical applications where the control valve may be required to be closed, without a blocking valve, for long periods of time with high differential pressure across the seating surfaces.

It requires special manufacturing, assembly and testing techniques. This class is generally associated with metal seat, unbalanced single-seat control valves or balanced single-seat designs with exceptional seat and seal tightness.

The test fluid is water at 100 psig or operating pressure

Leakage allowed is limited to 5 x 10-4 ml per minute per inch of orifice diameter per psi differential

Service dP at 50 to 125 oF

CLASS VI. This class establishes the maximum permissible seat leakage generally associated with resilient seating control valves either unbalanced or balanced single-seat with “O” rings or similar gapless seals.

The test fluid is air or nitrogen

Pressure is the lesser of 50 psig or operating pressure

The leakage limit depends on valve size and ranges from 0.15 to 6.75 ml per minute for valve sizes 1 through 8 inches, please check the below table for further details.

Control Valve Leakage Classification  Overview

as per ANSI Standard FCI 70-2

International Organization for Standardization (ISO)

The purpose of ISO standard 5208: Industrial valves – Pressure testing of metallic valves,

is the establishment of basic requirements and practices for pressure testing valves of various configurations that are used in general purpose, power generation, petroleum, and petrochemical or allied industry applications.

The intent is to provide a consistent set of procedural requirements and acceptance criteria that can be considered in conjunction with valve specific standards appropriate for specific applications.

Account has been taken of the valve testing requirement needs of EN 12266 and API 598 with requirements referenced for PN designated valves for the former and Class designated valves for the latter.

Notes:

The leakage rates only apply when discharging test fluid to the atmosphere.

The meaning of “No visually detectable leakage” is that there is no visible weeping or leakage in the form of drops or bubbles.

There is a loosely defined correspondence between the leakage rate acceptance values of API 598

and the leakage values Rate A as applied to DN ≤50, Rate AA-Gas and Rate CC-Liquid for other than metal seated check valves

and for check valves Rate EE-Gas and Rate G-Liquid. Rates A, B, C, D E, F and G correspond to values in EN 12266-1.

Inspection & Testing in accordance with BS EN 12266 – 1

Shell Test (Body Hydro Test):

Test pressure for each valve shall be a minimum of 1.5 times the allowable pressure at room temperature.

Test media: – Shell test shall be made with water contain corrosion or rust inhibitor.

Acceptance Criteria: – Visually detectable leakage from any external surface of the shell is not permitted.

Leakage from the operating mechanism sealing is permitted at the shell test pressure provided that there is no visual detectable leakage when the test pressure is 1.1 times the allowable pressure at room temperature.

Inspection & Testing in accordance with EN 12266 – 1

Seat Tightness Test :

Test pressure shall be a minimum of 1.1 times the allowable differential pressure at room temperature,

except that if the test fluid is gas, the test pressure may be lower of 1.1 times the allowable differential pressure at room

temperature or ( 6± 1) bar for valves:

Size up to DN 80 for all pressure ratings

Size above DN80 & up to DN200 for pressure ratings up to PN40 & up to Class300.

Acceptance Criteria: – As per below table.