Valve Inspection and Testing is an important subject for piping professionals as it ensures the integrity and performance of Valves during plant operation. During the shutdowns of operating plants, hundreds of valves tend to have various needs to be inspected. Also before using in new plants (during the design phase), the valves must be inspected thoroughly in manufacturer shop to ensure proper quality. API STD 598- Valve Inspection and Testing provides guidelines for inspection and testing of valves and the test is conducted by valve manufacturer. This Valve Inspection article will provide you with a few important points related to the inspection of valves and valve testing in the manufacturing shop as well as in operational plants.
Normally the following tests and examinations are performed on Valves/Components.
Required for all valve types. The shell test validates the strength and soundness of the valve pressure containing structure. This is actually a pressure test where normally air, inert gas, water, kerosene, etc can be used as the test fluid. The minimum shell test pressure that can used are listed in table 2 of API-598. The pressure is applied inside the assembled valve with ends closed. The valve shall be partially open and be able to hold pressure for a certain time. The pressure shall not be less than 1.5 times of maximum working pressure. No leakage is allowed.
Required for all valves that have the backseat feature, except for bellows seal valves. This is a feature that allows valve packing being replaced while it is in the service. Normally gate valve, globe, and parallel slide gate valves are subjected to this test. This pressure test is performed to verify leakage past the stem or shaft to bonnet seal i.e, backseat. Table 3 of API-598 provides required test pressures for the backseat test for different valve types. In this test, the valve shall be fully opened and the packing gland shall be loose or not installed. No leakage is permissible in the backseat test.
Low-pressure closure test/ High-pressure closure test
Closure tests are performed to confirm leakage past or through a valve’s closure mechanism. The Closure Test or Valve Seat Leak Test is performed after the successful completion of the Valve Body Test/shell test. One side of the valve inlet or outlet shall be subjected to the hydrostatic pressure and the amount of leak shall be measured on the opposite side of the valve. To explain it, if you are pressurizing valve inlet, then you have to measure the amount of leakage in the outlet. For both low pressure and high-pressure tests leakage through the disc, behind the seat rings, or past the shaft seals is not permitted. However, a limited amount of leakage is permissible at the seat-sealing surface interface that is listed in table 5 of API-598.
Double block and bleed (DBB) high-pressure closure test
Pressure shall be applied successively to each side of the closure and Leakage into the body cavity shall be checked.
Visual examination of castings
A visual examination must be performed on all castings to ensure conformance with MSS SP-55.
High-pressure pneumatic shell test
When specified in the purchase order, a high-pressure pneumatic shell test shall be performed. This test shall be performed after the shell test, using appropriate safety precautions. The pneumatic shell test pressure shall be 110 % of the maximum allowable pressure at 38 degrees C (100 degrees F) or as specified in the purchase order. Visible leakage is not allowed.
Table 1 of API 598 specifies the pressure test requirements for various types of Valves. The test equipment should not apply external forces that affect seat or body seal leakage. The following figure shows some sample test equipment used for valve testing.