Maintaining effective control of process media, whether in its liquid or gaseous form, is a fundamental concern for LNG operators. Many aspects of production depend on this, from ensuring operations are safe and environmentally sound to maximising output and profitability. 

The performance of valves is central here. They play an instrumental role in critical phases such as liquefaction, as well as managing the velocity and pressure of process media and protecting high value assets such as compressors.  

A strategic approach to LNG valve spares  

LNG valves need to provide reliable, repeatable process control in a wide variety of process conditions.  

Many applications involve extreme thermodynamics and pressure variations. This is accounted for in the selection of pressure-containing materials and the design of the valve trim (internal components which contact process media). Nevertheless, even the most highly engineered valves inevitably experience wear over time, and a plant’s entire valve population needs regular maintenance and repair. Maintaining a valve population is very much like a fitness regime: frequency leads to better health and efficiency which ultimately translates into better profitability.  

Managing this effectively to ensure the right valves receive adequate attention at the right moment is crucial. It demands a robust maintenance routine, closely aligned with the development and implementation of an intelligent spares strategy. Upfront decisions have a significant bearing on this. Having operational spares available on site prior to the commissioning phase ensures unforeseen issues can be navigated to avoid any delay to start-up and commercial operation.  

Getting the balance right is crucial; it can make the difference between enabling a continuous cycle of repeatable plant performance or triggering a downward spiral of decline. The latter leads to increased wastage, product/process variation outside acceptable parameters, lack of plant availability, unforeseen shutdowns and reduced profitability.  

There is no simple one-size-fits-all solution. The unique circumstances and characteristics of the individual plant need to be considered.  A plant like Yamal LNG, located within the Arctic Circle, is likely to have different operational challenges and priorities to Gorgon on Barrow Island. One faces extreme logistics challenges, constrained either by the ice road or shipping routes at different times of year and the other is situated within a Class A nature reserve in a sub-tropical zone. Factors like these influence operators’ priorities, which in turn impact best practice for valve spares.  

It is important to give valve spares proper attention, so decisions are purposeful and considered. This is not always easy in the midst of a new project, when there are so many factors to consider and reams of paperwork to be reviewed. But creating time and space to consider spares at an early stage can save a huge amount of trouble further down the line. Understanding the different types of valve spares is a good place to start. 

The three categories of valve spares 

Valve spares encompass consumables, hard parts and associated instrumentation – we will look at the three categories in more detail shortly. A best practice approach identifies critical valves and applications, as well as general maintenance considerations, to ensure the right quantities and types of each category are held onsite.  

The cost of procuring a certain type of spare should be considered alongside the potential repercussions of not having it when it is needed. In our experience, maintenance engineers tend to be more thorough and meticulous in routine valve inspections when they know a good stock of spares is held onsite. They are more inclined to open a critical valve, or a valve that isn’t quite performing as it should, at an earlier stage if they are confident that they will have the parts available to get it back online quickly if they find a problem.  

Ultimately, this rigour enables engineers to identify and act on opportunities to prevent the escalation of emerging valve problems. Issues linked to subpar valve performance can range from tripping the plant to the damage of major pieces of equipment such as compressors, separators and pumps. Valve failure can also result in serious consequences that present danger to life. Any of these scenarios can result in lost production if the plant has to shut down, and there are significant financial penalties attached to fugitive emissions, health and safety breaches or an inability to offload or export.   

When valve inspection or maintenance is delayed due to a lack of spares, vital opportunities to rectify issues at an early stage and avoid a disruptive event can be missed.  

Consumable spares 

A comprehensive stock of consumable spares is fundamental to rigorous and effective valve maintenance. Valve consumables include parts within the valve, such as gaskets, packing sets, metal seals and balance seals (depending on the valve design) as well as actuator repair kits. These parts all have a similar purpose, deforming under compression to form a seal. Any time a valve or actuator is opened for inspection, they must be replaced to ensure the integrity of seals is maintained.  

Seals play a vital role keeping valves operational as leaks can be costly and, in some cases, they can seriously compromise health and safety. So it’s vital for any plant to have an extensive onsite supply of these items to avoid a scenario where engineers delay inspection because they do not have the relevant consumables.   

It is imperative that OEM consumables are used as only the OEM knows the exact compression rates and critical dimensions to ensure a given part seals properly. Third party seals with incorrect compression or dimensions will result in leaks. This increases the risk of a negative environmental impact. Moreover, any upfront savings are a false economy if they lead to more significant costs due to damage from the leak paths that will occur. 

 

Hard spares 

LNG valves are often exposed to extreme operating conditions or circumstances that cause wear, erosion or corrosion over time. A valve’s internal trim is especially at risk.  

Trim damage can also be caused by a variety of issues including, but not limited to, phenomena such as cavitation or flashing that can occur due to pressure let down or the inherent physical properties of flowing media. 

The trim design of valves can be engineered to eliminate cavitation, but modifications to the surrounding pipework or evolving process conditions can introduce new vulnerabilities. Flashing cannot be eliminated, but materials are used to help the valve withstand it for as long as possible. Valves deployed in applications where flashing or cavitation occur should be subject to an enhanced maintenance routine. In this way, any wear can be identified and rectified at the earliest possible opportunity, before significant damage occurs and performance is compromised. 

Noise levels are also a concern for certain LNG valve applications. Again, in most cases the trim design can eliminate or reduce the potential for this. However, sometimes noise that does not originate from the valve itself can have an impact. If a noise issue is identified, valves need to be inspected as soon as possible since the associated vibration can cause catastrophic damage to a valve’s internals. 

Trim spares typically include the plug (usually an assembly of the stem and plug head), seat and cage. Different operators use different terms for the cage – essentially, it is an internal component that can perform multiple roles in the control of the process medium. These can include control of pressure let down and outlet velocity, as well as provision of valve characteristics such as linear or equal percentage etc. They can also clamp the seat in place and protect the plug.  

Additional hard spares include gland followers and guides (also called stem guides or guide bushes, depending on the valve design). Some valves also require retainers, especially if a soft seat and/or balance seal have been installed. In some valves, integral seat diffusers or liners protect the valve body from the exit velocity of the trim.  

As with consumables, when third party spare parts are used the operator carries the burden of the valve warranty being invalid and takes ownership of the valve. OEM parts are always preferable. They are manufactured to exacting critical dimensions that only the OEM fully knows and only the OEM can ensure support for the duration of the valve’s life. In certain circumstances – especially those relating to the key elements of a plant – use of third-party spares can invalidate the plant’s insurance. 

Understanding which valves are likely to experience problems resulting from issues such as flashing, cavitation and noise is key here. By ensuring relevant replacement parts are stocked onsite, engineers are given greater freedom and autonomy for preventative and corrective maintenance. This is a fundamental aspect of continual improvement. The alternative is to risk delaying inspection and maintenance, which can result in subpar valve performance hindering production levels. In the worst cases, this can cause irreparable valve damage leading to an extended or unplanned shutdown. 

For some operators, this risk is untenable, and a decision is taken to hold whole valve spares onsite for certain high-risk applications. This ensures that if a valve requires attention, it can quickly be taken offline and switched for the spare valve in the minimum amount of time to get the plant up and running again. Once the original valve has been removed for a complete overhaul, with parts replaced (ideally from the stored stock) or repaired as necessary, it is held as the spare in case of any future issues. This is not an uncommon scenario on LNG plants with multiple compressor trains.  

Instrumentation  

Valve instrumentation devices – such as positioners, air filter regulators, solenoid valves, lock up valves, pilot valves, volume boosters or quick exhaust valves – are typically mounted on the actuator. This makes them vulnerable to damage, both during installation and operation. They are also exposed to the elements, so they are susceptible to damage caused by humidity, ice, insects or sand depending on the plant environment. 

Since valves cannot function without these devices, best practice dictates that an optimised stock of complete units is held onsite.  

A tailored approach 

Ultimately, a valve spares strategy must balance the available budget against the potential production losses if a valve performs badly or has to be taken offline. There is no perfect solution to this challenge, but with a considered approach it is possible to manage risk effectively and ensure optimum process control is maintained.  

A detailed appraisal of critical and severe service LNG valve applications combined with an understanding of the plant’s unique circumstances and characteristics provides a solid foundation. This is where it can be advantageous to draw on specialist third party support guided by a Spare Parts Interchangeability Record (SPIR) to ensure optimum stock levels. It empowers maintenance engineers to work with confidence, taking proactive steps to keep valves in good working order to ensure continuity of plant availability and performance.