low mass disk make nozzle check as a fast closing ... Figure 2: Spring assisted and Low Mass Disk of ... Figure 6: Seat
Axial flow nozzle check valves for pumps and compressors protection
Nozzle Check valves which are also called axial flow check are very important for non-return fluid purpose with non-slamming and fast closing characteristics. They are mostly installed downstream of rotating equipment to protect the expensive mechanical equipment from possible damages due to backflow. Although they are costly option compared to other alternatives like swing and dual plate check valves but can save a lot through safe protection of costly rotating equipment and lower pressure drop. These valves are widely used in different sectors of oil and gas industry such as top site offshore platforms, subsea, refineries, pipelines, LNG and petrochemical plants. Advantages and design considerations of these valves are described in the next section.
Figure 2: Spring assisted and Low Mass Disk of Axial Flow Check Valve.
By Karan Sotoodeh, Aker Solutions
Advantages and design considerations •
Figure 1: Axial Flow Non Slam Check Valve.
January/February 2015
Quick closing and Easy opening disk: Short axial disk travel to the seat, spring assisted design in addition to low mass disk make nozzle check as a fast closing valve which is an advantage in critical lines with fast reversing flow. Fast closing response reduces the possibility of equipment damage due to back flow and provides good protection for expensive mechanical facilities. Low static pressure behind the disk in venturi
area makes a pressure differential over the disk causing easy opening of the disk as well. •
Robust Structure & Zero Leakage Robust body structure of the valve gives more resistant associated with vibration coming from upstream equipment (pumps and compressors) compared to dual plate and swing check valves. Integrated body without bolted body bonnet design reduces the leakage risk through the body close to zero. As a result of zero emission body characteristic, these valves are also used in subsea application. 1
E N D -U S E R F E AT U R E the valve as a result of venturi effect associated with nozzle design (bore reduction) avoids flow turbulences, minimizes erosion problems, maximizes flow capacity as well as easy opening of the disk. To sum up, this valve has advantages from both process and mechanical points of view. Figure 3: Robust design, Integrated body, ribs and axial nozzle casted together (Crain Design).
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Long valve life Non slam check valves have low or even no maintenance requirement from operational point of view. Having disk as the only moving part which travels very short distance to the seat makes wearing in the disk and the valve as minimum as possible. Spring is just exposed to axial load unlike the springs in dual plate check valves which increases the life and reduces the wearing. The valves are sometimes sent back to the manufacturers for examination and possible maintenance after quit long time of operation from
Figure 4: Integrated body non slam check valves (No bolted bonnet) (Crain Design). •
Low pressure drop and high flow capacity High pressure drop means higher wearing and erosion in the valve as well as more expensive pumps or
compressors selection to provide higher head and lower friction loss. The pressure drop for the valve is low and less than 0.1barg normally. In addition, smooth flow patern through
Figure 6: Seat is greased on the valve send back to Crain for maintenance.
Figure 5: Smooth flow in axial flow check valve and venturi (Bore Reduction) effect. 2
January/February 2015
E N D - U S E R F E AT U R E special geometry of nozzle in axial flow check valve makes venture effect and smoother flow characteristic as mentioned earlier in section3 so that just 2D or 3D of straight pipe upstream and downstream is enough. This saves material, cost and specially space which are critical parameters in offshore platforms and FPSOs.
7 to 10 years. Normally, these valves are opened, examined, seat is greased and pressure tested and sent back to the operator without any major maintenance. •
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Non slamming Although disk closing speed is fast, the disk closing action does not produce slamming against seat due to short stroke and spring assisted design which minimizes water hammer and valve slam in the liquid services. Water or liquid hammer is a possible operation problem in which a pressure wave is caused by the fluid service due to sudden closing or slamming of the valve. That can create noise, vibration and (Or) piping collapse due to additional piping loads. Shorter straight pipe run requirements Ideally check valves should have 5 diameters of straight pipe both upstream and downstream of the valves. Insufficient straight pipe before the dual plate and axial flow nonslam check valves especially when the fluid velocity is above 4.5 m/s causes significant wear in the valve and reduces the design life. Practically, achieving 5D of straight pipe due to layout space limit is very challenging and sometimes not possible. However,
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Tight shut off Axial flow check valves are tight shut off although they are metal seated. Even an erosive fluid can not disturb the sealing since the valves have robust designed. Normally they are pressure tested according to requirements of API 598 in which the leak through the seat should be maximum 3CC (Equal to 48Drops) per size in inch per minute. However, no or very small amount of leak is normally observed from the valves. Rarely clients ask for high pressure gas tests on the seat if the valve is installed in high pressure gas services.
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Different End to End design (Flexibility in End to End Design) Although manufacturer design (Short Pattern) is commonly selected for the valves, the compact design and ASME B16.10 (API 6D) (Long Pattern) end to end designs are also available in the
market. Compact nozzle check which meets requirements of API594 in end to end design can give higher pressure loss and more wearing internally compared to manufacturer end to end design. Unless otherwise space is an important parameter, compact design is not recommended. ASME B16.10 or API 6D (Long Pattern) could give pressure loss more or less the same as short pattern manufacturer standard. However, it is possible to order a new valve in replacement of the old valve with long pattern design from different non slam manufacturers (In case of maintenance) considering that manufacturer end to end design should be replaced by another valve from the same manufacturer.
Figure 8: Crain end to end (short pattern) design.
Figure 9: Compact Nozzle Check Valve.
Conclusion
Figure 7: Straight Pipe up and down of non slam Check valve Note1: The same straight pipe requirement for reducer or valve instead of elbow shown in figure 7. Note2: Upstream straight pipe is normally asked by vendors to be as long as 5D for Dual plate check valves.
January/February 2015
Selection of the suitable check valve in pumps and compressors discharges is an essential parameter in the mechanical equipment system design. The challenge is the valve should be opened easily with low pressure loss to make movement of the flow smoothly and to be closed gently and fast to avoid flow return to the nonoperated mechanical equipment. Non slam axial flow check valves are the best choice of valve selection in this application which provide high energy saving and safe equipment protection. 3
