Check Valves Design Features

This post is initiated to give some basic information regarding Check valve design features, it uses, standards, types, advantages, disadvantages etc.

Check Valves

Another type of valve commonly used in conjunction with other valves is called a check valve.  Check valves are designed to restrict the flow to one direction.  If the flow reverses direction, the check valve closes.   Check valves look different but each performs the same basic function: flow thru one direction and no flow (check) return. Check valves are designed to prevent backflow. Fluid flow in the desired direction opens the valve, while backflow causes it to close. Usually an arrow on the outside of the valve indicates the flow.

Check valves are commonly installed on the discharge side of the pump. The most important role of a check valve is to act as the automatic shut off valve when the pump stops to prevent draining of the system, which the pump fills. However, each check valve has different shut off characteristics. The following brief study of these characteristics will help you to select the best check valve for the job.

Pressure Surge (Water Hammer) can be greatly controlled by selecting the correct check valve i.e., Silent Checks shut off very fast, 1/20th to 1/10th second, making excellent surge protection devices when pumping short distances less than half a mile. Conversely, the Automatic Control Check Valve has extremely low (and variable) speed shut off; an excellent surge protection device when pumping long distances (more than half a mile).

No single check valve is all serving. For example, the Lift Check Poppet (Center Guided) would be excellent for clean fluid, but a poor selection for waste water because of the obstructive flow passages.

Check valves are available in numerous types, styles and shapes. Following are six (6) basic types of check valves. Of these basic types, three (3) are available in several forms:

Swing check valves are the most commonly used being designed to cause the least resistance to flow. They are not recommended wherever frequent reversal of flow occurs. Horizontal lift check valves are used in applications where the flow is subject to frequent irregular or frequent flow reversals. They have much less tendency to slam in a fluctuating current.

Selection Criteria

There are four concerns, which should be addressed when selecting a check valve;

• Non-slamming characteristics
• Head loss characteristics
• Cost
• Application

A close interrelationship exists between the first three. It might be stated that: consider three parts which equal a whole where x = head loss characteristics, y = non-slamming characteristics and z = cost. Then,

x + y + z = 100

In the equation if you add to x, an equal amount must be subtracted from y or z or a combination thereof. Check valve design is, as a rule of thumb, similar to the equation. The design engineer has two approaches; Design a valve, which takes all three into equal consideration or Place Emphasis on one or two of the criteria. By taking both approaches a variety of valve designs become available enabling the specifier to choose a valve tailored to the fourth selection criteria application. Having a good understanding of each of the criteria will help the specifier choose the best valve for a given application.

A check valve needs a minimum flow velocity through it to ensure that the door is fully open and stable. If the valve is too large, the velocity will be too small the door will not be stable in the flow and rapidly wear out. The minimum velocity required depends on the design of valve used which is dependent on the duty it is being asked to perform. Standard valves (Eurocheck & 6BS) operate between 2 and 3.5 m/sec, whilst valves situated near pumps (6NS & 8RD) require 3.5 – 5 m/sec.

Non-Slamming Characteristics

There are two major criteria, which affect a check valve’s potential for slamming and creating surges. One is the amount of time it takes the valve to close. The second is the way in which the disc travels from the open to the closed position.

Closing Time

When a pump shuts down, the forward momentum, or velocity of the flow diminishes. When the forward flow stops, a reverse flow will begin. Depending on system conditions, the reversal can happen quickly with rapid increases in velocity occurring. The longer the reverse flow is allowed to go unabated, the more momentum (velocity) it will build. If the reverse flow is 1.) allowed to build momentum and 2.) suddenly stopped, slamming (water hammer) will occur with a resultant pressure surge. To avoid contributing to slamming and surges a check valve must close very rapidly or very slowly. If closed fast enough, a check valve can virtually eliminate reverse flow, keeping slamming and surges to a minimum. Silent Check Valves are known for their ability to do so. More often, a rapid closing check valve does not eliminate reverse flow but keeps the momentum (velocity) to a minimum, thereby minimizing the slam and surge. Closing the valve slowly allows the reverse flow to build momentum but avoids a sudden stoppage. Once again, this will minimize slamming and the accompanying surge.

Rapid Closure

Three factors will impact the speed at which a check valve closes. The first is the velocity of the reverse flow. The second is the use of mechanical assistance through springs, weights and levers, cushions, etc. The third is the length of the disc’s stroke, or how far the disc must travel to reach the closed position.

1. Velocity of Reverse Flow: As indicated previously, it is inadvisable to allow the reverse flow to close the valve as slamming and surges will occur. If slamming and surges are a concern this is precisely the situation to avoid.
2. Mechanical Assistance: Rapid closure of a check valve can be achieved through the use of springs, weights and levers, etc. Two types of check valves, silent checks and dual disc (sometimes called a double door), included springs in their original designs. Springs, weights and levers, etc. were added to swing checks over the years as a way to overcome the valves propensity to slam. The use of mechanical assistance to close a valve rapidly will, to a varying degree, depending on the valve, succeed. However, it is often done at the expense of efficiency as turbulence and head loss are increased, sometimes significantly.
3. Length of Disc Stroke: The length of the disc stroke or how far the disc must travel to reach the closed position is of particular importance. Obviously, the shorter the stroke the faster the valve will close. Valves with discs designed to travel in a linear motion1 (Silent Check Valves) typically provide a short stroke.

The length of stroke can vary from 35° to 90° on valves with a non-linear (see Disc Travel below) stroke. A shorter stroke on a nonlinear valve is usually achieved by placing the seat on a 45°-55° angle shortening the distance the disc must travel to reach it. If the body geometry is designed to provide full flow coupled with a non-turbulent path, this design will provide a good combination of non-slamming characteristics and low head loss.

Slow Closure

As stated previously, a valve designed for slow closing will allow the momentum (velocity) of the reverse flow to build. In this scenario, the speed at which the valve closes is calculated and controlled to avoid slamming and surges. In many circumstances it is necessary to control only the final 10% of the disc travel. Naturally, the length of the disc stroke will affect the speed as with the rapid closing valve.

There are several ways to control the closing speed of a slow closing check valve. Dashpots, oil accumulators, cushions, and power actuators are just a few. The specifier should work closely with the valve manufacturer to determine the best valve and method of control to utilize.

Disc Travel

There are two basic ways in which a check valve disc travels. The most common is a non-linear motion. The second is a linear motion. Check valves utilizing a non-linear motion have a pivot from which the disc swings or rocks. The pivot is located at the top (Swing Flex, Swing Check), center (Dual Disc), or eccentrically offset (Tilted Disc). A check valve utilizing a linear design (Silent Check Valve) places the disc in-line with the seat. The disc travels in a straight line from the open to closed positions. In a check valve whose disc travel is linear, the percent of flow area will be equal to the percent of travel of the disc.

Percent Open = Percent of Flow Area

If the valve disc is open 5% the flow area through the seat will also be 5%.As the valve reaches its closed position there is minimal reverse flow present to slam the valve closed. Now consider a valve with a nonlinear stroke. In nonlinear designs, disc position is not equal to flow area. This is particularly true in the final stages of disc travel prior to closure. With the disc at 5% open the flow area could still be as high as 20-30%. This means that at closure a reverse flow of large volume is still present and will slam the valve closed with significant force.

In summary, it could be stated that length of disc travel plus type of disc travel equals a check valves potential to slam.

Head loss

There are two givens regarding head loss. First, all check valves create head loss and second, Head loss costs the user money through increased power consumption by pumps. The importance of head loss varies. In a pressurized distribution system it’s an issue of considerable concern. In a gravity waste water collection system it may be of little concern.

Reducing Head loss Through Design

Three design factors will impact the amount of head loss created by the valve. They are total flow area through the body, body geometry and how the disc interacts with the flow.

Body Design

To avoid head loss the total valve body area, minus the internal components, should be equal to, or slightly greater than, the mating pipe diameter. Body geometry is equally important. A design which creates turbulence in the flow will in turn generate head loss. The design should avoid abrupt changes in flow direction and the body should have a contour which allows the flow to take as straight and smooth a path as possible. Extended or globe style bodies are especially beneficial as they return the flow to normal in a gradual manner.

Disc Design

Head loss and a check valve disc is somewhat like the old cliché about real estate; Location, Location, Location. If the disc is located in the flow it will create head loss. As discussed earlier under rapid closing, springs, weights and levers etc. will improve a valve’s non-slamming characteristics. However, it is done at the expense of head loss. Placing a lever and weight and/or spring on a swing check valve forces the disc into the flow, creating turbulence and head loss. If slamming and head loss are of concern and a swing type valve is desired, it would be better to shorten the stroke by placing the seat on a 45° angle (Swing Flex) instead of the typical 90° angle. This will allow the disc to ride above the flow instead of being forced into it while supplying good non-slam characteristics.

If a silent check valve is selected for its superior non-slam characteristics the specifier should consider the body design as discussed on Page Six, Number One. Is the body expanded to allow for full flow? Is it contoured to provide minimal turbulence?

As one can see, there are trade-offs to be made when selecting check valves. The specifier must weigh the importance of each selection criteria and make an informed selection on the type of valve best suited for the application. Once selected, it is necessary to review the designs of the various manufacturers to be assured of getting the best valve possible.

Cost

The specifier must look not only at the initial purchase price but two other factors as well, the cost of energy consumption by pumps due to head loss and maintenance costs.

Energy Consumption

It is a simple matter to compare the head loss of two check valves and compute the difference in energy savings and dollars. A lower head loss valve will probably cost more to purchase. However, the difference in cost is often realized within a year or two by the savings in energy consumption. From that point on it will pay dividends on a daily basis.

Maintenance Costs

System downtime, valve location and the cost of parts and labor should all be considered. Saving a few dollars on the initial purchase can lead to the loss of thousands if a backhoe has to be brought in to dig up a valve or a critical system has to be shut down.

Selection

Use of the following Check Valve Comparative Selection Chart coupled with an understanding of the criteria will provide you with a way to make an informed selection.

Check Valve Comparative¹ Selection Chart

NOTES:

1. The ratings do not reflect quality. They simply reflect a valve types relative head loss and slamming characteristics when compared to others. The ratings do not reflect a particular manufacture but rather a generic type of valve.
2. The valve with the least head loss was rated at one, the highest at nine. All others were given relative positions between one and nine.
3. The valve with the best non-slam characteristics was rated at one, the worst at nine. All others were given relative positions between one and nine.

Instructions On Selecting A Check Valve

1. Select from the Check Valve Comparative Selection Chart those valves recommended for the application.
2. Plot the selected valves coordinates on the graph.
3. The valve plotted closest to the bottom left hand corner will offer the best combination of low head loss characteristics and non-slamming characteristics.
4. If one characteristic is more important than the other, you may wish to make a different selection. The chart will show what is being sacrificed to make the alternative selection.
5. Next, consider the cost. Write in the cost of each valve next to its plotted coordinates. If the selection you made in Number Four is within your budget you are all set. If not, you may want to choose an alternate.
6. Caution: Remember, the higher the head loss of the valve selected the higher your pumping costs will be. You must consider both purchase and operating costs when making a selection.

Lift Check Valves

Horizontal-Lift Check Valves

They have an internal construction similar to a globe valve. They should always be used in the horizontal position. The disc is equipped with guides above/ and or below the seat and is guided in its vertical movement by integral guides in the seat bridge or the valve bonnet.

Vertical Lift Check Valves

This is almost similar to the horizontal lift check valve, namely a free floating guided disc that rests when inoperative on the seat. They can be used only in vertical pipe lines.

Ball Check Valves

Instead of a disc in these type of valves a ball serves the purpose. This type has been found to be well suited for manufacture and operation in plastics materials.

Piston Check Valves

Wafer Type Check Valve

This is for installation between two flanges and has a helical spring loaded disc and is is of very compact design. Because of its special design it is considered as non slam type and can be used in the vertical and horizontal positions.

There is another type where a hinged type of disc is used. The closing is performed with separate springs for the two half’s.

Swing Check Valves

Applications

• Onshore and subsea pipeline
• Petroleum refining
• Oil and gas processing
• Chemical processing
• Power generation
• Gas transmission
• Marine
• Pulp and paper
• Corrosive / Erosive conditions
• High Temperature conditions

Design

• One-piece & Two-piece clapper designs
• Thru-conduit or Full opening
• Accessories available for Position Indication & Slam Prevention
• Multiple installation positions available
• Available with reverse flow capabilities
• Optional removable seats & high temperature trim applications available. A variation is the tilting disc valve where the disc is hinged slightly above the center of gravity of the disc.
• They are so named because of their particular mode of operations. They are the most widely used because of their low flow resistance and simplicity of operation. The disc is hinged and the seats are machined in the tilted wall opening. The discs can be finished with metallic or non metallic finishing depending upon the flow characteristics. To increase sensitivity to the flow an outside lever and weight van be attached to assist in the valves operation.

Typical Installations

• Wellhead injection lines
• Production flow lines
• Sales gas export facility
• Heat exchanger
• Compressor Train
• LNG storage

Swing Check Valves, 2-Inch and Smaller for domestic services

MSS SP-80; Class 125, cast-bronze body and cap conforming to ASTM B 62; with horizontal swing, Y-pattern, and bronze disc; and having threaded or solder ends. Provide valves capable of being reground while the valve remains in the line. Provide Class 150 valves meeting the above specifications, with threaded end connections, where system pressure requires or where Class 125 are not available.

Swing Check Valves, 2-1/2 Inch and Larger for domestic services

MSS SP-71; Class 125 (Class 175 FM approved for fire protection piping systems), cast iron body and bolted cap conforming to ASTM A 126, Class B; horizontal swing, and bronze disc or cast-iron disc with bronze disc ring; and flanged ends. Provide valves capable of being refitted while the valve remains in the line.

Vertical Flow Swing Check Valves

Crown Swing Check Valves can be supplied for use in vertical flow-up or vertical flow-down service conditions.  It is necessary to specify the intended orientation upon the placement of the order to ensure the valve operates properly and meets the customer’s operating requirements.

Vertical Flow-up Service:  Valves already in service can be field modified to operate properly in the vertical flow-up orientation with the addition of a vertical flow pin.  They can also be ordered directly from Crown Industries in this configuration.

Vertical Flow-down Service:  Valves to be installed in the vertical flow-down orientation must be ordered directly from Crown Industries as field modifications cannot usually be performed to convert the valve to this required orientation.

Bronze Swing Check Valves

The specifications for a bronze swing check valve is given below.

Specifications as applicable (depending on valve size, pressure rating and end connection)

Swing Check Valves – Accessories

Counterbalance Arms and Weights (Counter Weight Assembly)

Counterbalance Arms and Weights can be provided to assist in either valve closing or valve opening, or when the valve is being installed in the vertical flow-down position.

Valve Closing:  When the counterbalance assembly is utilized in valve closing, the counterbalance assembly is designed and orientated to ensure a positive shut-off.

Valve Opening:  When the counterbalance assembly is utilized in valve opening, the counterbalance assembly is designed and positioned to counteract a portion of the clapper assembly weight to allow the clapper assembly to float on the flow stream, thus reducing the pressure drop through the valve.

Vertical Flow-down:  For valves installed in the vertical flow-down position, the counterbalance assembly is designed and positioned to assist in returning the clapper to the closed position ensuring a positive shut-off.

When a counterbalance assembly is required, it is recommended to specify the intended positioning to ensure proper orientation and valve operation.

Lever Lock Assembly (Lock Open Assembly)

The external Lever Lock assembly provides a means for the customer to hold the clapper in the full open position (out of the flow stream) during the passage of pigs and spheres, during normal operating conditions when the customer wants to eliminate clapper movement (interference) or during reverse flow operations.

The external Lever Lock assembly should not be used to lock the clapper assembly in the closed position as damage to the valve components can result.  For special instances (very low pressures), the Lever Lock assembly may be utilized to hold the clapper in the closed position for short periods of times.  However, this should not be attempted without first consulting the factory.

An external Lever Lock assembly is strongly recommended during the passage of pigs and spheres, particularly during the passage of smart pigs.

Rovane Slam Retarders

Crown Rovane Slam Retarders provide a dependable and efficient torque for controlling clapper slam and flutter.  Long trouble-free service is assured through the simple and rugged construction and the basic engineering premise of the slam retarder.  It can also be equipped with a limit switch to indicate the clapper open position at any angle.

Proximity Switches

A Proximity Limit Switch can be used to provide an input signal to a process control with either an open or closed indication.  Designed and built to rugged specifications for use in the most industrial and environmental applications, each switch is totally enclosed, hermetically sealed and capable of operating in hazardous locations.

Sub-sea Check Valves

Design Features

• Ring Type Joint (RTJ) cover seal capable of withstanding external and internal pressures
• Lever type lock open with integral wrench to remove gland cover and lock valve in full open position
• Option worm gear operator lock open to lock valve in full open position.  Worm gear operator can be fitted with a hand wheel for diver operation or with a R.O.V. interface for R.O.V. operation
• Choice of Gland designs / Shaft seals
  • Standard: O-ring primary shaft seal with secondary POLYPAK seal
  • Optional: Spring energized PTFE Lip seals
  • Optional: PTFE “Vee” type packing that can be externally energized with   plastic packing to seal a potential shaft leaks
• Optional conduit clapper to assist in passage of pigs, spheres and inspection equipment
• Optional angled seating face on clapper and removable seat to assist in the passage of pigs, spheres and inspection equipment during reverse flow operations
• Choice of Cover seal materials
  • 316 Stainless Steel
  • Soft Iron Zinc Plated
  • Additional materials available upon customer request
  • Soft Iron Epoxy coated
• PTFE (XYLAN^TM) coated studs, nuts and gland bolting to resist corrosion and aid in installation and removal of bolting
• Optional cap for bolting to protect exposed threads from sea water environment and aid in installation and removal of bolting

Maximum Protection in Corrosive and Critical Situations

To accommodate severe service conditions, a variety of tough products and materials has been developed. For instance, in corrosive and erosive applications, the clapper and the mating seat faces on swing check valves can be overlaid with Stellite, Alloy 625 (UNS NO6625), 316 or 410 Stainless Steel.

In extremely corrosive applications, Alloy 625 and Duplex Stainless Steel (UNS-S31803) are available to provide maximum protection.

Metal-to-metal clapper seals and metal bonnet seals allow valves to operate under these extreme temperature conditions(-75°F to 650°F).

Wafer Check Valves

Wafer Check Valves for domestic services

Class 250, cast-iron body with replaceable bronze seat, and non-slam design lapped and balanced twin bronze flappers and stainless steel trim and torsion spring. Provide valves designed to open and close at approximately one foot differential pressure.

Features of wafer check valves

• General purpose check valve where weight and size are limiting factors
• Is quickly and easily installed between flanges
• Horizontal or vertical flow up
• Cost-effective
• Reduced port, non-piggable valve

The use of wafer type check valves may not be acceptable in certain applications, such as underground feed-mains. Review use with applicable insurance and approval authorities prior to making commitment.

In horizontal piping, first loosely install the four lowest Stud Bolts and Hex Nuts to support the Valve. Next, insert the Flange Gaskets and then install the remaining four Stud Bolts and Hex Nuts. Center the Valve within its mounting flanges and tighten all Hex Nuts uniformly using a cross-draw sequence.

Powered Pump Check Valves

Powered Pump Check Valve offers significant advantages over a gravity or spring-operated valve.

Typical applications

Typical applications of powered Pump Check Valve systems are municipal installations with long runs, hills or anywhere there are major changes in elevation. Pump Check Valves are also ideally suited for industrial process systems such as the primary cooling towers in the power generation industry.

Advantages of Pump Check Valves

The Pump Check Valve reduces the possibility of water hammer throughout the system by controlling valve opening and closing speeds so the operation does not cause pressure surges in either direction. The Pump Check Valve performs all the necessary functions on a pump installation; it serves the purposes of both a swing check valve and an isolation valve, saving both cost and space.

Advantages of the powered version

All pump installations require flow controls to check or stop back-flow through the pumping system. This can be accomplished through the use of a gravity or spring-operated pump check valve or a power actuated pump check valve.

Gravity or spring-operated check valves open and close with flow changes. An increase in flow pushes the valve open, a decrease allows gravity or spring force to close the valve. Because their operations are not controlled, these valves can stick or operate sporadically, which causes flow surges. Often, their flow capacities are low and they do not give tight shutoff.

Class 1 Check Valves / Non Slam Check Valves

Now a days, Class 1 check valves are used in many industries, being below requirements,

• Class 1 check valves shall undergo regular Maintenance.
• Regular check valves are designed for pressure rating, however class 1 / Non Slam check valves are designed for flow conditions.
• Class 1 check valves are Inspected and tested for functionality, however regular check valves are not checked.
• Numbering for class -1 check valves are unique and identified in P&ID’s and even in critical valve lists.

Dynamic Flow Testing Of Check Valves

In a piping system, the shut down of a pump or the closure of a control valve, can cause the flow in the system to reverse. Check valves are used to prevent flow reversal, but due to the inertia and friction of the check valve components, limited flow reversal will still occur. Regardless of the initial velocity of the flow (as long as the check valve is fully open) the reverse velocity at which the valve closes is dependent only upon the valve geometry, mass of the valve, and the deceleration of the flow. At a constant rate of change of velocity, or deceleration, the reverse velocity is independent of the initial velocity as long as the initial velocity is greater than the minimum velocity at which the valve is fully open.

The sudden closure of the check valve at a reverse velocity can cause large pressure surges downstream of the check valve and negative pressures upstream of the check valve. However, the magnitude and duration of the pressure surges are as much a result of the piping configuration as the check valve. The resulting upstream and downstream transients are typically calculated with a set of complicated equations or with a transient computer program (Tullis 1989). The calculations require information in the form of the relationship between the deceleration of flow and the reverse velocity at which the check valve closes.

Previous studies had concluded that valve geometry affected the magnitude of pressure surges and reverse velocities. The conclusions were:

• • Reverse velocities and pressure surges are greater for valves with a larger mass of valve components.
  • Reverse velocities are greater for valves with larger strokes or travel of components to close.
  • Reverse velocities are less for valves that were spring assisted to close.

These conclusions are justified because of the increased time necessary to accelerate and overcome the inertia of valve internals and the distance they must travel.

Tests of the 10-inch and 12-inch valves, and piping configuration tests of larger check valves have produced the additional conclusions:

• Reverse velocities will be greater for valve designs with larger flow coefficients.
• Reverse velocities will be greater for valves with increased friction in the valve shafts, guides, and internal components.
• The slope or orientation of the pipeline has a significant effect on reverse velocities. An upward slope will usually cause a decrease in reverse velocity, and a downward slope will increase the reverse velocity.
• The wear of flow components can also have a significant effect on reverse velocity. Wear can cause additional friction or can effect the alignment and position of valve components.

To overcome the mass or inertia of the valve internals, the reverse flow must develop sufficient flow forces (pressure drops) to close the valve. Valves with larger flow coefficients require larger reverse flows to produce the same pressure drops or flow.