The slanted seat check valve also offers enhanced resistance to water hammer. The valve has a double eccentric shaft position as well as an increased seating angle. This yields a shorter valve stroke, thus reducing the time taken for the door to close.
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Hydraulic dampers are recommended, especially when the valves are installed on a pumping station where high frequency opening and closing of the valve is required. The valve disc closes quickly over the first 85% of its angular travel before meeting the hydraulic damper. The damper then dissipates the kinetic energy of the disc and forces it to open slightly. The disc closes until it contacts the damper again and this cushions the disc until it returns to its fully closed position, sealing the valve. This function greatly reduces the onset of water hammer due to the damped and controlled method of valve closure.Hydraulic dampers are recommended, especially when the valves are installed on a pumping station where high frequency opening and closing of the valve is required. The valve disc closes quickly over the first 85% of its angular travel before meeting the hydraulic damper. The damper then dissipates the kinetic energy of the disc and forces it to open slightly. The disc closes until it contacts the damper again and this cushions the disc until it returns to its fully closed position, sealing the valve. This function greatly reduces the onset of water hammer due to the damped and controlled method of valve closure.
For more details, please see our slanted seat check valve product information.
Nozzle check valves are designed with the valve disc connected to the stem which is guided on the central horizontal axis. A spring is positioned between the disc and the diffuser sleeve. When flow enters the valve, the hydraulic force exerted onto the front face reacts against the spring, causing the spring to compress and allow the valve to open. When the flow stops, the spring forces the disc to return to the closed position.
Due to the spring-assisted closure and the short linear valve stroke, the nozzle check valve is one of the quickest acting check valves available and is commonly used in pumped systems where water hammer is a potential concern. Because the disc is constantly in the direct line of flow, the head loss characteristics of this valve are higher compared to that of conventional swing check valves.
For more details, please see our nozzle check valve product information.
In order to choose the right check valve for your application, several selection criteria should be considered. First, however, there is not one type of check valves being the best choice for all applications, and the selection criteria may not be equally important for all cases. Some of the things you may need to consider are fluid compatibility, flow characteristics, head loss, non-slam characteristics and total cost of ownership.
All check valves referred to in this article are designed for water and treated wastewater applications, but using the valves for raw wastewater/sewage applications may cause some issues. When selecting a valve for these fluids, you should consider how the presence of solids may potentially affect operation of the valve.
If a check valve closes very fast, it may prevent slamming against upstream equipment such as pumps. However, the rapid closure will not protect against the surges caused by pumps being started and shut down. If the valve opens (and closes) quickly, the flow will change rapidly and thus increase likeliness of surge occurrence.
Head loss is, among other things, a function of fluid velocity. The head loss through a valve is determined by the internal design of the valve and the opening degree. When a valve is designed with a restricted (narrowed) opening compared to the pipeline, the velocity will increase through the valve, increasing the head loss as a result. Vice versa, if a valve is designed with less restriction and bore is equal size to the pipeline, the head loss will be smaller, and the valve will in practice not affect the overall head loss of the system. There are a number of values for head loss, amongst others zeta values, Kv and Kvs values.
The costs for your check valve consist of more than just the purchase price. For some installations, the most important costs may be purchasing and installation, but in other cases, maintenance or energy costs may be equally or even more important. Also protecting more valuable equipment such as e.g. pumps has to be considered, and looking at the valve performance will be crucial. When considering costs as a selection criterion for your check valve, the total costs over the life of the valve should be considered. In general, the simpler the valve construction is, the lower are the maintenance requirements. The higher the Kv value, the lower the energy consumption. The higher the performance, the better the protection ability.
Check valve slam can affect pressure surges in a negative way. First step of the process is when the pump stops, starting the pressure surge. Second step is, when the flow is reversed, slamming against the fully closed check valve. If the check valve closes too fast, the kinetic energy is turned into high pressure, stressing the pipes, and causing high noise.
A slam sounds like if the disc or the ball from the check valve is hitting the seat and can make quite some noise. However, the sound is not caused by the physical closing but by a sound wave arising from a pressure spike stretching the pipe wall.
To prevent the occurrence of check valve slam, the valve should close in a controlled way and slower when near closed position. For a check valve to close slowly, it requires additional ancillary equipment, such as hydraulic dampers, which act as a cushion to the valve door, as it comes into its seated position. This slower closure allows the fluid to pass through the check valve until it closes, causing less kinetic energy turning into high pressure, and thus less energy to feed and maintain the surge. Consideration must be given to the upstream pump to ensure that it is suitable for reverse spin and flow.
As swing check valves have the disc in the flow stream, helping with rapid closure, they have better non-slam characteristics. However, today many pumps are frequency converted, enabling them to adjust the start-up and closure time to avoid water hammer.
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Tilting Disc Check vs Swing Check Valve
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(Chemical)
(OP)
10 Aug 07 05:12Can anybody explain me the main disadvantage of tilting Disc Check valve over a swing Check valve?
(Mechanical)
10 Aug 07 16:29tilting disc check valves are a lot smaller and lighter so for a given pipe size they don't cost as much. Since the tilting disc is partially balanced, the cracking pressure is less-Particularly for vertical upward flow. Neither will work well for vertical downward flow, and neither will be happy if they are not installed right-side up.
(Mechanical)
12 Aug 07 19:01The characteristics of the tilt check valve are such that the potential for waterhammer are reduced. Not as much as you would see with an annulus type non slam check valve.What you aim for is a valve with a low mass and short travel so that is is closed before the returning pressure wave will slam it shut. The tilt check by its geometry is better than a swing check valve. the latter has been described as "agricultural at best".Refer Fluid Transients in Pipeline Systems ARD Thorley. Also check out the Delft Laboratory site. Delft have tested check valves for dynamic performance. Als see www.pipingdesign.com where there are papers on surge.
Geoffrey D Stone FIMechE C.Eng;FIEAust CP Eng
www.waterhammer.bigblog.com.au
(Mechanical)
13 Aug 07 04:49I think the tilting disc check valve's life is longer then the common swing check valve. As the damage from disc is smaller then the common swing check valve.
(Mechanical)
13 Aug 07 08:57Seconding what stanier said, as I recall the tilting disc is airfoiled so it goes to fully open position and stays there. A swing disc valve continually bounces around and ultimately the pin wears which mat make it hang open, leak due to misalignemnt, or the pin may even fail. If the disc goes downstream it inevitably finds something expensive to jam.
(Mechanical)
13 Aug 07 10:58I support the replies from both JimCasey and Stanier and the descibtion that swing checks are "agricultural at best".
During the NEL valve workshop in Aberdeen (October ) there was an interesting paper about Maintenance, Inspection and Testing of Non-Return Valves. This paper was presented following a near accident on a British platform where 12 out of 12 swing check valves in a production line failed.
However, Priya, correct valve selection should always be based on the application. For standard non-return applications a tilting disc or dual plate check valve will do. For more critical applications (e.g. to protect expensive rotating equipment or potential risk of waterhammer) an axial or non-slam check valve type is recommended.
(Mechanical)
17 Aug 07 14:02What I hate most about tilting disc check valves is the restricted accessibility for maintentance.
The body of a swing check valve typically is equipped with a separate flanged cover plate. This allows you to access the flapper for maintenance or to remove the flapper for line testing and flushing while the valve remains in installed condition.
Tilting disc valves, however, typically have a split body. You have to remove the entire valve body for maintenance. It is even worse where tilting disc checks valves are equipped with butt welding ends. In this case the pipping manifold has to be spread apart for maintenance. Sometimes this requires the provision of additional break flanges.
(Mechanical)
18 Aug 07 00:16Grandnobi,Of course if you used an annulus type non slam valve such as Noreva or Mokveld you wouldnt need to be continually taking your check valves apart for maintenance.The prime cost of a piece of mechanical equipment may only be 12% of its whole of life cost if it is a high maintenence item. Great if you want job security in maintenance but if you are the owner and want reduced life cycle costs and plant availability avoid the swing check and the dual plate type valves.There are two types of damage by surge. The first is obvious and something breaks. The second is the damage from repetitive events that cause fatigue failure of seals, glands, discs, seats, shafts and gaskets. When one piece of equipment finally succombs to this type of failure you are then reliant on the standby which is in a parlous state itself having been subjected to the same fatigue history.Design the system for whole of life not the maintenance crew's career path and longevity!
Geoffrey D Stone FIMechE C.Eng;FIEAust CP Eng
www.waterhammer.bigblog.com.au
(Nuclear)
15 Nov 07 17:49If your velocity is not high enough to hold the disc fully open both the swing check and tilting disc valve will experinece bushing/hinge pin wear. Once difference between tilting disc and swing check is that the TD is very intolerant to any wear and will result in seat leakage. Swing checks with moderate hinge/bushing wear may still seat tightly at high DP.
If your velocity is below 10-12 fps water, expect disc oscillation for a swing or TD valve. These are ballpark numbers and differ based on the specific desings.
(Mechanical)
15 Nov 07 18:59CWFCNuclear,Great insight of what industry already knows. Given the nuclear industry's routine documentation of failures is there a Report you can reference?
Geoffrey D Stone FIMechE C.Eng;FIEAust CP Eng
www.waterhammer.bigblog.com.au
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