Common Problems

• In conventional wedge gate valves, fluid force during intermediate disc travel imposes a moment on the disc that tends to cause disc tipping, which in turn is responsible for high edge loading and damage to the disc and seat faces as well as the lower guide surfaces as shown in Figure 4-11. The fluid-induced moment on the disc for any given flow and ∆P condition is zero in the fully open and fully closed positions with a maxima at an intermediate disc travel position. The magnitude of the fluid-induced moment on the disc and the potential for damage increases with an increase in flow velocity. Under high energy blowdown conditions, damage to the disc and seat faces and/or the guide surfaces has been observed with many conventional wedge gate valve designs and parallel disc designs [5.55].

Figure 4-11

Typical Seat and Guide Damage Locations in Conventional Flexible Wedge Gate Valves Under High Flow Conditions

• Normally open valves with high turbulence flows (such as downstream of pumps, control valves, orifices, strainers, and elbows) may be subject to high wear rates due to the turbulence-induced motion between internal components. In solid and

flexible disc valves (with single-piece discs), wear typically occurs at two locations:

(1) stem head and disc T-slot and (2) body and disc guides.

In other gate valves with multiple-piece discs, additional wear can occur between the disc components. Excessive wear can cause valve failure, including stem separation from disc and disc sticking at an intermediate position.

• Seat leakage is a common problem in gate valves and can be caused by several factors:

— Insufficient wedging loads

— Sediment or scale in the seat area

— Disc and seat erosion

— Wire drawings or steam cuttings caused by high flow velocities between the disc and seat

— High pipe loads and moments, especially with low pressure class valves

— Excessive wedging forces which cause high deformations

— Reversed installation of unidirectional valves (for example, double-disc valves installed with flow in the non-preferred direction)

• Stem packing leakage is a common problem.

• Leakage through the bonnet flange is also a frequent problem. Body-to-bonnet joints utilizing a spiral wound gasket may exhibit leakage if gasket surfaces are not

cleaned properly or have not been properly dressed. Occasionally, the dimensions of the joints do not provide for proper gasket compression, or the bolts are not torqued properly.

• The increase in the required opening thrust under pressure locking and/or thermal binding conditions can cause the valve to fail to open. The common modes of failure to open under pressure locking and/or thermal binding conditions include

insufficient actuator output and failure of the weak link (in the valve or the actuator).

• Stem thrust may become smaller under higher disc friction loads due to increase in stem factor or stem coefficient of friction. This phenomenon (called rate of loading effect or load sensitive behavior) was observed during EPRI’s testing, NRC-sponsored testing at INEEL, and valve operations in nuclear plants. These tests show that conversion of actuator output torque to stem thrust became less efficient at higher thrust levels. For the same actuator torque switch setting, the stem output thrust under high ∆P condition can be lower by as much as 25% of its value under static conditions. The rate of loading effect must be accounted for in evaluating required stem thrust under load.

• Static and fatigue failures in internal valve and actuator components can occur due to excessive stem thrust values. In particular, during programs to verify MOV design basis capability, the opening/closing thrust levels for many valves had to be increased significantly to ensure MOV capability. During in situ testing and control switch activities, some valves and actuators were inadvertently overloaded beyond their thrust ratings. Failures include broken stems, stripped stem threads, broken or severely deformed gate T-slots, and bent or broken guide rails. Such failures can be prevented by appropriate stress/fatigue analyses of the weak link components.

• In some service situations, isolation valves may stay in one position for long periods of time. If left in the open position for a long time, deposits and particulates can accumulate in the gate guides and recesses of the valve, preventing full closure and possibly resulting in damage to the disc or seat if the valve is forced closed.

• Threads often bind due to corrosion and foreign matter, especially in gate valves with inside threads. Outside threads also become corroded and crusted with

deposits, but the deposits are easily seen and can be removed to make the valve operable.

• The use of gate valves in throttling service is a basic misapplication, and this practice usually leads to damage of the valve or the valve seats.

• Some valve vendors underpredicted thrust and torque requirements for some gate valves by underestimating friction coefficients, flow effects, and metal-to-metal interactions.

• Some MOV problems are caused by overestimation of motor actuator output torque/thrust capability.

• Weak link failure can be caused by under-predicting the actuator output thrust, which can be caused by overpredicting stem friction coefficient (stem factor) or ignoring inertia overshoot.

• Many valve problems are caused by improper maintenance and/or repairs. For example, elastomeric and non-metallic components can be damaged by improper solvents and cleaners. The use of counterfeit and low-quality, commercial grade spare parts can also cause valve failures.