Why Are Large-Diameter Valves Difficult to Open and Close?

Users of large-diameter globe valves often report a common problem: when these valves are used in high-differential-pressure services, such as steam or high-pressure water, they can be difficult to close while the system is pressurized. Even when considerable force is applied to the handwheel, leakage may persist and the valve may be difficult to shut tightly.

This problem is primarily attributable to the valve’s structural design and the fact that the torque a person can generate manually is limited.

The maximum horizontal force an average adult can exert is generally around 60–90 kgf, depending on the individual’s physical condition.

Globe valves are generally designed for flow from below the disc to above it. To close the valve, an operator applies horizontal force to the handwheel, causing the disc to move downward. During this operation, the operator must overcome the combined effects of three forces:

The total operating torque is expressed as:

∑M = (Fa + Fb + Fc)R

As the valve size increases, the axial thrust force also increases. Near the fully closed position, this thrust can approach the actual line pressure acting across the valve, because during closure, P1 − P2 ≈ P1, with P2 = 0.

For example, consider a DN 200 globe valve installed in a steam line operating at 10 bar. The axial thrust force alone—the first component to be overcome during closure—is given in the source as:

Fa = 10 × πr² = 3,140 kgf

The horizontal force required to close the valve can therefore approach the maximum force a person can normally exert. Under these operating conditions, it can be extremely difficult for one person to close the valve completely.

Some plants recommend installing this type of valve in the reverse direction to make closing easier. However, this can create the opposite problem: the valve may then be difficult to open after it has been closed.

Large-diameter globe valves are commonly installed at boiler outlets, main steam headers, and steam mains. These locations can present several operating challenges.

The differential pressure at a boiler outlet is often relatively high, resulting in higher steam velocity and more severe erosion of the valve sealing surfaces.

In addition, boiler combustion is never 100% efficient. Moisture may therefore be present in the steam leaving the boiler, potentially contributing to cavitation-related damage to the valve sealing surfaces.

Steam immediately downstream of a boiler may experience intermittent superheating. As the steam approaches saturation, if the boiler water has not been adequately softened or treated, certain acidic or alkaline substances may precipitate and cause corrosion and erosion of the sealing surfaces.

Other substances capable of crystallizing may also accumulate on the valve sealing surfaces. Such deposits can prevent the valve from achieving a tight shutoff.

Steam demand downstream of a steam header may fluctuate with production requirements. Significant changes in flow velocity can readily lead to phenomena such as flashing and cavitation, which may erode and damage the valve sealing surfaces.

Large-diameter pipelines generally need to be warmed up before being brought fully into service. During warm-up, steam is typically admitted at a low flow rate so that the pipeline heats up gradually and uniformly. The globe valve can then be fully opened once the pipeline reaches the required temperature.

This procedure helps prevent rapid temperature increases and excessive thermal expansion that could damage pipeline joints or connected components.

However, the valve opening is often very small during warm-up. Under these conditions, erosion can be considerably more severe than during normal operation, significantly shortening the service life of the valve sealing surfaces.