LST Longer Service Trim: A smarter solution for low-flow control valve damage

That philosophy sits at the heart of Longer Service Trim (LST), the latest innovation developed through IMI's Growth Hub, our engineering incubator focused on solving complex customer challenges. Designed specifically for applications that require extended periods of operation at near-minimum flow conditions while maintaining full operating range, LST helps protect critical valve components. It also extends maintenance intervals and improves process reliability.

For plants under pressure to maximise uptime while reducing maintenance costs, it offers a practical solution to a long-standing industry problem.

Low-flow control is an operational decision made by plants, often to protect other critical equipment in the system. Low flow rates are challenging for valves because they increase the risk of damage to the valve trim from concentrated flow at the sealing point. This type of valve damage can occur gradually, but it often happens quickly and degrades valve performance in both the control and the shut-off/sealing element.

When a valve is operating near its minimum controllable flow, fluid velocity can increase dramatically through small internal flow passages. This creates conditions that accelerate wear mechanisms such as cavitation, erosion, internal leakage and damage to sealing surfaces. Over time, these effects reduce valve performance, increase maintenance requirements and can ultimately lead to premature trim failure.

In plants where low-flow conditions occur frequently and maximum-flow conditions are required on the other end, the valve itself is protected. LST technology solves this problem by leveraging the valve expertise IMI has built over many decades.

It is common for industrial plants to work through the different operating ranges of their systems. This is common in power generation, chemical processing, and petrochemical applications, where process conditions fluctuate, and equipment protection is critical.

In power plants, for example, valves may routinely operate at less than 30% of their rated capacity to protect rotating equipment and minimise thermal or pressure shocks. In other facilities, valves may spend significant periods in low-flow operation because they were originally sized for different operating conditions or future capacity requirements. While these approaches may be operationally necessary, they can create significant challenges inside the valve.

Every control valve has a minimum controllable flow limit. Below this threshold, tiny clearances between the plug and cage can become the primary flow path. In these conditions, a valve may technically remain open even when it is expected to provide a tight shut-off. The result is often concentrated high-energy flow directed towards critical seating surfaces, where damage accumulates rapidly.

For many plants, the consequences are familiar: declining performance, increased leakage and frequent trim replacement.

Operators managing severe service applications have historically faced a difficult choice. Because high-velocity fluids naturally create cavitation and erosion, many sites accept that damage is inevitable and schedule trim replacements at fixed intervals. In some cases, maintenance teams replace trims every six months simply to maintain acceptable performance and leakage rates.

While this approach keeps processes running, it also incurs ongoing costs due to frequent planned outages. Increased maintenance time means a greater need for replacement parts and maintenance labour, reduced process efficiency, as well as increased operational risk for facility personnel. Keeping your teams out of the plant as much as possible is, of course, critical.

The question is not whether cavitation and erosion can be eliminated completely. In many severe service applications, they cannot. A more effective approach is to control where that damage occurs.

Our Longer Service Trim was developed around a simple engineering principle: if cavitation and erosion cannot be entirely avoided, their impact should be redirected away from the valve's most vulnerable components.

Using advanced fluid dynamics and proprietary design concepts, LST relocates the highest-energy impact zone away from critical sealing surfaces, significantly reducing wear where it matters most.At the centre of the design is a specially engineered plug nose incorporating labyrinth-style grooves. Although the labyrinth section is only approximately 10mm long, it fundamentally changes how flow behaves during low-opening conditions. The labyrinths themselves have no impact at high flow rates, making this technology ideal for valve upgrades. Basically, it solves a core problem without sacrificing other performance attributes.

The design shifts the vena contracta (the point where flow reaches its smallest effective area and highest velocity) away from the seating surface. By controlling flow in this region, LST reduces the destructive impact typically experienced by the seat and sealing areas. The technology’s material composition is designed accordingly:

Plug (including spindle) and seat ring: 440C SS + HT, 316SS + stellite

Cage and stem: 410SS + HT, 316SS

Balance cylinder: 410SS + HT, 316SS or forged material equivalent to body

The design provides additional plug guidance during low-opening operation while reducing clearance Cv under gap-control conditions. As the valve opening increases, flow naturally transitions through the cage passages as intended, maintaining normal operational performance across the wider operating range. The result is a significant reduction in damage to critical sealing surfaces, helping preserve valve integrity for substantially longer periods.

For operators looking to improve reliability and reduce maintenance demands, LST delivers several measurable advantages:

By relocating the primary erosion and cavitation zone, LST can extend the operational life of your internal trim components from approximately six months to more than two years in suitable applications.

The enhanced design enables you to achieve more stable control over a wider operating range, helping your facility maintain performance under varying process conditions.

Longer trim life reduces the frequency of your operations team’s planned shutdowns and maintenance interventions, helping you lower lifecycle costs while increasing equipment availability.

Protecting critical seating surfaces helps you maintain valve performance for longer, reducing the risk of leakage and unexpected failures.

An ethylene production facility in China experienced recurring problems associated with low-flow operation in a temperature-control application.

Actual plant operating conditions had moved beyond the original design specification, causing an IMI 840/860 critical water control valve to operate at less than 10% opening for extended periods. The resulting cavitation and erosion caused severe internal leakage and frequent maintenance interventions. After reviewing the application, our engineers recommended installing Longer Service Trim.

Following implementation, the valve achieved significantly improved operational performance from June 2024 onward. Turndown capability increased from approximately 10:1 to 30:1 while erosion and cavitation effects were redirected away from the valve's critical sealing surfaces.

The result was a more robust solution capable of supporting both low-load and higher-flow operating conditions. LST improved reliability and reduced maintenance requirements.

Control valve performance is rarely determined by a product alone. The most effective solutions combine application knowledge, fluid dynamics expertise and a deep understanding of the operating environment.

Longer Service Trim demonstrates how targeted engineering innovation can solve the type of persistent challenges that many operators have long accepted as unavoidable. Whether implemented as a trim upgrade within an existing valve or as part of a complete valve replacement programme, LST offers a practical way to reduce wear, extend component life and improve plant profitability.

Discover LST Longer Service Trim

If your operation is experiencing cavitation, erosion, or premature trim wear in low-flow applications, IMI's Longer Service Trim could provide a more reliable, cost-effective solution.

Learn more about the LST Longer Service Trim Control Valve Upgrade and discover how innovative engineering can help improve your plant performance, extend maintenance intervals and increase operational resilience.