SPOTLIGHT – BACK SEATING ON API600 WEDGE GATE VALVES
March 14th, 2018 by Amanda Schumann
Removal of Packing Rings and use of Backseat: –
Most APV valves are designed with stem ‘backseats’. However, valve manufactures in general including APV do not recommend the practice of backseating due to the inability of determining the effectiveness of the backseat seal. In addition, backseating is not designed to be absolutely drip tight. Due to the inherent risks, backseat should never be used to allow gland packing replacement or repair while the valve is pressurised.
The primary reason for the backseating facility is to prevent wear of the packing rings.
When placing a new valve into service, Australian Pipeline Valve recommends a preliminary packing adjustment to verify proper packing load. Additionally, it is recommended that a Baseline Leakage Test be performed following installation, but prior to start-up.
During the packing life cycle, normal and routine maintenance of the packing arrangement must be administered. Normal cycle life will typically require 5 to 8 packing gland nut adjustments. Torque values vary depending upon valve size. Refer to the Packing Bolt Torque chart at the APV website for the recommended torque values. Tighten the packing nuts clockwise to compress the packing. Do not over tighten or the valve will become too tight to turn (see 5.2.1). Fugitive emission stem packing can be fitted to reduce leakage rate to 100 PPMv for 1,500 cycles (APV cast valves are fugitive emission prototype tested). Removal of old packing should be done in an experienced workshop. Using a special flexible removal tool. The removal tools have special hooks, which screw into the packing ring. Removal of the packing ring is a difficult and time-consuming operation. Care has to be taken not to scratch the stem of the walls of the packing chamber during the removal of the packing rings.
Depending on pressure, service, age, quality of valve, valve design and company procedures, the issue of backseating can range from minor to catastrophic. Also, many cheap valves say they have backseating but don’t! For a full set of APV IOM’s click here. You will find valuable information for Oil & Gas valves irrespective of what brand you have in service.
Here is an extract from the APV IOM….
Gate valves should only be used in the fully open or fully closed position.
CAUTION: Gate valves should not be left in the fully ‘back seated’ position under normal operating conditions. The packing may dry out under these conditions and leak as the valve is closed.
A cool valve may leak through the gland when opened to hot fluid. Wait before tightening the packing as the problem may go away.
Proper safety equipment and apparel should be worn when preparing to service a valve. Observe the following general warnings:
Tools Required: – aside from standard wrenches (for bonnet cap screws and packing gland nuts) the only special tool needed for minor Australian Pipeline Valve valve maintenance is a packing hook.
Packing: – special care is to be placed in the tightening of the gland nuts during installation, to ensure the proper packing adjustment and functionality.
The packing gland should be checked periodically in service and tightened as necessary to stop leakage around the stem. Tighten in a manner to develop even loading on the gland. Tighten only enough to stop the leak.
For normal operation in the open position, the stem should be backed off so that the backseat is not in contact. This permits the stem packing to assume it’s intended sealing function and not conceal unsatisfactory stem packing. In the event of stem packing leakage, the back seat can be used to stop stem leakage until circumstances permit a system shutdown and time for packing replacement. Stem packing replacement with the valve under pressure and backseated represents a hazard and should not be undertaken. The hazard is magnified as fluid pressure or temperature increases or when the fluid is toxic.