Valve Pressure Testing: API 598 Explained — Shell, Seat & Backseat Tests

API 598 'Valve Inspection and Testing' is the standard that governs factory pressure testing of gate valves, globe valves, ball valves, butterfly valves, plug valves, and check valves after manufacture and before shipment. It defines three mandatory test types, the test fluid, test pressures as multiples of the rated working pressure (CWP), minimum hold times, and allowable leakage rates. Understanding API 598 is essential for procurement engineers reviewing valve datasheets and test certificates.

The Three API 598 Test Types

1. Shell Test (Hydrostatic Body Test)

The shell test verifies the structural integrity of the valve pressure boundary — body, bonnet, end connections, and cover plate. Test pressure: 1.5 × CWP (Class Working Pressure at 38°C) for the valve's pressure class. Test fluid: water (hydrostatic). The valve is filled with water at test pressure for the minimum duration specified in API 598 Table 2 (e.g., 15 seconds for ≤DN50 valves, 60 seconds for DN600+ valves). Acceptance criterion: no visible leakage or structural distortion. Note: for valves ordered to ASME B16.34, the shell test pressure equals 1.5 × the rated pressure at the maximum temperature rating (which may be lower than the CWP formula for alloy steels).

2. Seat Leakage Test

The seat test verifies the sealing performance of the valve's closure element (gate, ball, disc, plug) against its seats. API 598 defines two seat test types:

• Low-pressure air test (2.4–6.9 bar / 35–100 psi): used for soft-seated valves (PTFE, PEEK, elastomer seats) to detect gross leakage. Measured by observing bubble rate in water bath or with detection fluid.
• High-pressure liquid test (1.1 × CWP): used for metal-seated valves (gate valves, globe valves, metal-seated ball valves). Measured by drops per minute or bubble rate at the downstream side.
• Acceptance criteria — leakage rates: API 598 Table 3 defines allowable leakage by valve type and seat material. Soft-seated ball valves: 0 drops/min (zero leakage). Gate valves (metal seat): up to 40 drops/min for smaller sizes. Globe valves: varies by size.

3. Backseat Test

The backseat test is performed on gate valves and globe valves that have a backseat — an internal seat between the stem and the bonnet that allows the packing to be replaced under pressure when the valve is fully open. Test pressure: same as shell test (1.5 × CWP). Test fluid: water. Acceptance: no visible leakage. Not all projects require a backseat test — it must be specifically called out in the purchase order.

API 598 Test Pressures by Valve Class

Reading a Valve Test Certificate

A valve test certificate (or pressure test report) to API 598 will typically show:

• Valve identification: tag number, purchase order number, serial number, size, pressure class, material (body / trim), end connections
• Test type: shell, seat (LP air or HP liquid), backseat — and which were performed
• Test pressure: actual pressure applied (psi or bar)
• Test fluid: water (hydrostatic) or air (pneumatic)
• Test duration: actual hold time in seconds
• Result: PASS / FAIL + leakage observed (drops/min or ml/min or bubbles/min)
• Test date, inspector name and signature, QC stamp
• API 598 Edition: ensure the certificate references a current edition (latest is API 598 10th Edition, 2022)