New valve significantly reduces operating costs

Oene Roorda, SMX International, looks at check valve options available to prevent return flow, and advances in design now achieved to optimise performance and minimise operating costs:

Check valves are a vital component of almost any process plant, preventing return flow by closing as quickly as possible when flow reverses. They perform a critical function in protecting plant and mechanical equipment such as pumps or compressors, where return flow would damage the equipment by driving it in reverse, resulting in unnecessary shutdown of the system or, in severe cases, the whole plant, but may also be necessary to enable reciprocating pumps and compressors to function, for example, or to stop reverse flow in lines feeding a secondary system in which the pressure can rise above that of the primary system.

The variety of check valve types, and significant differences in performance even within one type, can mean wide variations in plant maintenance requirements and life cycle costs, amounting to $millions over the typical 15 year design life. Now, however, a new design takes check valve performance to a new level, significantly reducing operating costs.

Key considerations

While the performance required of a check valve will vary with the system, two factors are fundamental considerations in check valve selection. Firstly, when flow reverses, the valve should close quickly to prevent return flow from gaining momentum. Otherwise, the formation of significant reverse flow velocity will introduce an undesirable high pressure surge (known as ‘hammer’) on sudden shut-off and/or cause heavy ‘slamming’ of the closure member against the seat. If the hammer effect is large, this can cause fatigue damage to the piping itself, as well as loosening joints, introducing broader plant maintenance issues. This becomes an increasingly significant issue with higher density fluids, such as water, and on larger diameter piping, as the larger the fluid mass, the greater the potential hammer effect may be.

Secondly, a check valve should ideally allow flow in the desired direction with as little resistance or pressure loss as possible. This will minimise the pump or compression action and therefore energy consumption required, and impacts significantly on the cost of valve operation.

While the function and requirements are common, the various types of check valves have correspondingly varied performance characteristics. The swing check valve, the most common type, features a disc that ‘swings’ closed through a long 90-degree arc (horizontal to vertical). Some reverse flow will occur before shut-off is achieved and closure will result in severe slamming and damaging hammer, particularly in high density fluids. Further, pressure loss across the valve is significant because, when open at normal or higher flow rates, the disc will flutter in the flow. This flutter can also cause undesirable vibration of the piping system.

A variation comes in the dual-plate check valve, featuring two ‘half’ discs instead of one (reducing mass and travel distance to closure). This is quicker to respond to reverse flow with reduced slam, but the discs will still flutter in the flow, creating resistance. Over time, this can cause spring fatigue and wear to the moving parts, resulting in early valve failure and raising reliability issues. Other alternatives include a tilting disc check valve (similar to a swing check valve, but with the disc at an angle) which is quicker to close, minimising reverse flow, and has mild-slam closure characteristics, as well as ball, lift, and nozzle check valves.

A nozzle check valve has a disc which moves axially in the flow, and closing is spring assisted. The traditional nozzle check valve features a valve disc, shaft and bearings, compression spring, and diffuser. Flow in the desired direction lifts the disc off the seat towards the diffuser. When fully open the disc sits stable against the diffuser, with any cessation of forward flow causing the spring to close the disc back against the seat ring, preventing reverse flow.

Compared with other check valve types, this offers the fastest dynamic performance and lowest pressure loss, but not all nozzle check valves take advantage of its inherent superior design potential, and there have remained areas for design optimisation to further improve performance, which have now been addressed in the very latest design. Focusing on the two fundamental criteria, the primary emphasis has been on improving dynamic performance and minimising pressure loss.

Optimised performance

A key factor in improving dynamic performance has been capitalising on the Venturi effect, achieved using computational fluid dynamics (CFD) to optimise the internal flow contour. The Venturi effect creates a strong low pressure field behind the valve disc, adding a large hydrostatic component to the opening force. Optimising this has allowed the introduction of a novel and powerful dual-spring closing action (patent pending), with the installation of a secondary spring with five times the force of the single spring employed in traditional nozzle check valves, which assists in overcoming the static friction in the bearings during the initial stage of valve closing.

This powerful dual-spring closing action is combined with an ultra-short valve stroke (a 25% reduction on existing nozzle check valve designs), minimised mass of the moving parts, and ultra-low friction metal precision bearings. Finite Element Analysis (FEA) has been used to develop an ultra-light webbed valve disc which, together with a hollow shaft, results in an approximate 50% weight reduction of the moving parts compared to existing designs. Further, the metal bearings are coated with a high-tech PVD solid lubricant coating, reducing the friction to 25% of traditional Teflon bearings. The result is a faster dynamic performance than was previously thought possible, closing rapidly before any appreciable reverse flow occurs, thereby minimising slam, hammer effects and pressure transient through the piping system, with long-term maintenance benefits.

Minimising life cycle cost

A further aspect of the design optimisation lies in minimising hydraulic or pressure loss. Initially a second-generation nozzle check valve incorporated an improved flow profile to reduce pressure loss. Now, a third-generation design combines significantly faster dynamic performance and extremely low pressure loss. Importantly, lower pressure loss will increase transmission efficiency, reducing compressor fuel consumption, with environmental benefits and minimised 15-year life cycle costs of the valve (15% lower than traditional nozzle check valves, 75% lower than swing check valves). The incremental fuel cost of the compressor arising from pressure loss across the valve is the largest component in a check valve’s life cycle cost.

Existing nozzle check valves have a few common design deficiencies. In the full disc design, for example, the seal ring location disrupts the flow contour at the point of highest velocity, and the ribs holding the diffuser (integral with valve body) are located where flow velocities are relatively high. Now, in the latest design, the diffuser contour has been optimised to improve its ‘aerodynamic’ characteristics. The use of CFD in the design process allowed multiple design iterations to be analysed and evaluated relatively rapidly, and recirculation zones, which are largely responsible for hydraulic losses, to be minimised. The consequent highly efficient diffuser results in a high degree of pressure recovery and exceptionally low pressure loss.

Importantly, the very fast dynamic non-slam response and very low pressure loss characterising this latest design results in superior protection of pumps, compressors and other components from destructive return flow and hammer (with broad plant maintenance benefits) coupled with reliable, maintenance-free operation, and the lowest life-cycle costs of any check valve currently available. Whereas life cycle costs of other check valves are typically 3.5 to 4 times the capital cost, the life cycle costs of this latest design non-slam nozzle check valve from SMX International are typically no more than 2.5 times the capital cost; a substantial and valuable saving for any process plant operator.