Oene Roorda looks at check valve options to prevent return flow, and at the advances achieved in the latest non-slam nozzle check valve design, now in production
Check valves perform a critical function in preventing reverse flow and protecting plant and mechanical equipment. But the variety of different types, and significant differences in performance even within one type, can mean wide variations in through-life maintenance requirements and life cycle costs, amounting to $millions over the typical 15 year design life of piping components. Now, however, a next-generation non-slam nozzle check valve has taken check valve performance to a new level, significantly reducing operating costs.
A vital component of almost any pipeline system, a check valve's duty is to prevent return flow by closing as quickly as possible when flow reverses, to protect mechanical equipment in a piping system. This is particularly critical in the case of compressor and pumping systems, where return flow would cause damage to the equipment by driving them in reverse, resulting in unnecessary shutdown of the system.
When flow reverses, the check valve should close quickly to prevent return flow from gaining momentum. This is important because 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 on larger diameter piping, as the larger the fluid mass, the greater the potential hammer effect may be, making an effective and responsive check valve all the more important.
Ideally, a check valve will also allow flow in the desired direction with as little resistance or pressure loss as possible, since this will minimise the pump action and therefore energy consumption required, and impacts significantly on the through-life cost of the valve.
Check valve variety
While the function and requirements are common, the various different types of check valve have correspondingly varied performance characteristics. The most common type is the swing check valve, which requires a disc to close through a long 90 degree arc, in horizontal hinges, allowing some reverse flow to occur, and with closure that results in severe slamming and damaging hammer. 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.
Other variations include a dual-plate check valve (quicker to respond, in vertical hinges, but still with slam on closure and with disc flutter creating flow resistance, and wear to the spring and moving parts which can result in early valve failure), and a tilting disc check valve with mild slam closure characteristics. Among other types are ball check valves, lift check valves, 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 a diffuser. Flow in the desired direction lifts the disc off the seat towards the diffuser. In the fully open position the disc sits stable against the diffuser, with any slowing or 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, the nozzle check valve has the fastest dynamic performance and lowest pressure loss.
Not all nozzle check valves take advantage of its inherent superior design potential, however, and there have remained areas for design optimisation to further improve performance. Initially a second-generation nozzle check valve was produced incorporating an improved flow profile to reduce pressure loss. Now, a third-generation nozzle check valve has been developed, optimising the design to produce a faster dynamic performance than was previously thought possible, and very low pressure loss. This in turn results in superior protection of pumps, compressors and other plant components from destructive return flow and hammer, coupled with reliable, maintenance-free operation, and the lowest life-cycle costs (LCC) of any check valve currently available.
A number of the very latest advanced tools have been utilised to optimise the design of this new Non-Slam Nozzle Check Valve from SMX International. These have included computer flow modelling (computational fluid dynamics, CFD), as well as computer stress modelling (finite element analysis, FEA), and flow testing (using rapid prototype development and flow loop testing), both to improve dynamic performance and reduce hydraulic losses.
Maximised dynamic performance
First priority has been to improve on the dynamic performance of existing designs, as the key parameter. This has been successfully achieved by optimising the Venturi effect (using CFD to optimise the internal flow contour to achieve this); an ultra-short valve stroke (a 25 per cent reduction on existing nozzle check valve designs); and minimising the mass of the moving parts and ultra-low friction metal precision bearings. FEA has been used to develop an ultra-light webbed valve disc which, together with a hollow shaft, has resulted in an approximate 50 per cent weight reduction of the moving parts compared to existing nozzle check valve designs. Further, the metal bearings are coated with a high-tech PVD solid lubricant coating which reduces the friction to 25 per cent of traditional Teflon bearings.
A novel and powerful dual-spring closing action (patent pending) is key to the valve's ground-breaking dynamic performance. Traditional nozzle check valves employ only one spring and do not take full advantage of the Venturi effect which is inherent to this valve type. In designing this latest valve, the optimised Venturi effect (which creates a strong low pressure field behind the valve disc, adding a large hydrostatic component to the opening force) allowed the installation of a very strong secondary spring. The powerful dual-spring closing action, low inertia, ultra-low friction in the bearings and ultra-short travel distance of the disc result in this valve's excellent dynamic non-slam response. It closes rapidly before any appreciable reverse flow occurs, minimising slam, hammer effects and pressure transient through the piping system.
Minimised pressure loss
A further aspect of the design optimisation lies in minimising hydraulic or pressure loss. Lower pressure loss will increase the transmission efficiency, reducing compressor fuel consumption, with environmental benefits and reducing the life cycle cost (LCC) of the valve. The incremental fuel cost of the compressor arising from pressure loss across the valve is the largest component in the check valve's LCC.
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 that hold the diffuser (integral with valve body) are located in the middle of the diffuser where 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, simulating fluid flow through the valve and minimising recirculation zones which are largely responsible for hydraulic losses. The consequent highly efficient diffuser results in a high degree of pressure recovery and exceptionally low pressure loss (pressure loss coefficient K=0.45).
Additional aspects have also been optimised in the design of this next-generation non-slam nozzle check valve, offering a number of benefits that address recognised issues in terms of application suitability, procurement, and maintenance. For example, unlike traditional nozzle check valves which have an integral diffuser and body, the latest valve features a separate diffuser body. Further, the springs are installed in the hollow shaft and are replaceable without major valve disassembly. The all-metal assembly with no elastomers makes the valve readily suitable for high temperature applications.
And the metal-to-metal seal (providing a tight shut-off) and robust self-lubricating metal bearings with ultra-low friction coating guarantee maintenance-free, reliable operation.
This latest non-slam nozzle check valve design has potential applications in natural gas, LNG, and oil pipelines, including subsea applications, as well as refineries, and petrochemical plants among others, and is suitable for horizontal and vertical installation.
Moreover, the result of this next-generation nozzle check valve design is not only superior performance, and effective protection of mechanical equipment but, importantly, minimised life cycle costs. Whereas life cycle costs (LCC) of other check valves are typically three and a half to four times the capital cost, the LCC of this design non-slam nozzle check valve is typically no more than 2.5 times the capital cost; a substantial and valuable saving.
Oene Roorda is a director of SMX International Inc, based in Ontario, Canada. www.smx-international.com