Manual Check Valves

In the functional grouping of the entire valve family, the check valve will be found in the group defined as valves designed to control the direction of flow. Check valves differ considerably in their construction and operation from the other groups of valves, designed either to stop the flow entirely (gate, plug, and quick-opening valves), or throttle the flow to a desired degree (globe, angle, needle, Y, diaphragm, and butterfly valves).Check valves are entirely automatic in their operation, and are activated internally by the flow of fluid or gases which they regulate. Check valves permit the flow of fluids or gases in only one direction; if the flow stops or tries to reverse its direction, the check valve closes immediately and prevents a back-flow. As soon as the pressure in the line is re-established, the check valve opens and the flow is resumed in the same direction as before.

Basic Designs of Check Valves

There are three basic designs of the check valve: the swing check valve, the horizontal and vertical lift check valve, and the ball check valve.

Swing check valves are the most popular of all check valves since they offer very little resistance to the flow when in wide open position. They are generally used on all piping where the pressure drop is of prime importance. Swing check valves are used for handling liquids, and can be installed in vertical or horizontal position without impairing their performance. However, these check valves are not recommended for such applications where the reversal of flow is frequent, since this causes the valve disk to fluctuate rapidly and result in "valve chatter."

Horizontal lift check valves are quite frequently assembled on the same valve bodies as those used for the regular globe valves. They are generally used for such applications where the reversal of flow and pressure fluctuations are very frequent, since they have less tendency to develop "disk slam" and valve chatter. Horizontal lift check valves are used for handling steam, air, and gases on horizontal piping lines, and they are not recommended for installation on vertical piping systems.

Vertical lift check valves are similar in construction to horizontal lift check valves, and are especially designed for installation on vertical piping systems. Another modification of the vertical lift check valve is the angle vertical check valve, which is used on right angle turns in the piping systems.

Ball check valves are designed to handle viscous fluids, and for services where scale and sediment are present. These valves, usually made in vertical, horizontal and angle designs, are particularly recommended for rapidly fluctuating lines because of their quiet operation. During the ball check valve operation, the ball rotates constantly, equalizing the wear on the ball and seat; thus prolonging the life of the valve.

Some of the swing check valves have external lever and counterweight balance arrangement, to make the valve disk more sensitive to the flow and open under a minimum of fluid pressure. Further design variations of the check valve include the stop check valves and non-return valves.

Stop check valves sometimes called "screw-down" check valves, are actually modifications of the globe or angle valves. This modification consists of making a slip stem connection to the valve disk instead of using the disk locknut. In this design, the disk can be closed by hand, but can be opened only by the check valve action; i.e., by the fluid pressure under the disc. Probably the most common application of the stop check valve is in safety non-return valves. The ASME Boiler Code specifies these valves for the boiler nozzle of every boiler, when two or more boilers are connected to the same header. These valves are also called boiler stop check valves, or boiler "screw-down" checks.

Size, Temperature, and Pressure

Bronze check valves are usually manufactured in sizes from 1/8 to 3 in., with steam working pressure ranges from 125 to 200 psi - for water, oil and gas. The joints are usually screwed or soldered. Some of these valves also have flanged joints. The temperature limit is 550°F, by ASTM Standard.

Cast iron check valves are available in small sizes from 1/4 to 2 in., and in large sizes from 2 to 12 in. Steam working pressure of these valves ranges from 125 to 250 psi. Small cast iron check valves usually have screwed connections, and large valves flanged connections. However, some of the large cast iron check valves also are available with screwed connections. The temperature limit for these valves is 450°F, by ASTM Standard.

Cast steel check valves are available in small sizes, from 1/4 to 2 in., and in large sizes, from 2 to 8 in. Steam working pressure range is from 150 to 2500 psi, with the temperature limit from 500°F to 1100°F, by ASTM Standard, depending on the alloy used.

In addition to these basic materials listed above, check valves are also made of stainless steel, Monel metal, nickel, PVC and other corrosion-resistant materials to withstand the corrosive action of the fluids being handled. However, there are no major deviations in the basic design.

Seat and Disk Angle Design

Since the pressure producing the flow in the pipe line must be sufficient to lift the check valve disk from its seat, designers have used various seat angles to aid this lifting action. The most commonly used seat and disk angles are 0, 6, 12 1/2, and 45 deg.

In the 125 psi class, the disks of swing check valves usually will be found at an angle of 6 deg. to the vertical. In the 200 psi and up class, the disks are usually placed at 45 deg. angle, since sufficient pressure is available to lift the disk and open the flow in the line. Horizontal disks, placed at 90 deg. angle to the vertical, are found in lift check valves.

Application of Check Valves

General surveys have shown that check valves rank third place in actual applications in plants, factories, and industrial use. Gate valves usually rank first, with globe valves second, and check valves following immediately behind globe valves. Since check valves are completely automatic, they require very little attention and are real work-horses of the valve family. In view of the fact that comparatively few check valves are used on one line, it is a good rule to install better quality or higher pressure-rated check valves than the more numerous gate and globe valves on the same line. This policy will result in better service, with less attention and repairs in the future.

Inspection Practice

While on the line, check valves can be visually inspected for external leaks. If leaks are present around the pipe connections, cap or plugs, the check valve must be removed from the line and repaired. When there is no pressure in the line, some of the check valves can be inspected and repaired without removing them from the line. On some lift-check valves the entire valve mechanism is accessible by removing the valve cap. If the disk, seat, and body are covered with sludge, dirt, foreign matter, or if the operation of the disk is sluggish or binding, the check valve should be removed from the line, thoroughly cleaned and repaired. The seat and disk contacting surfaces can be inspected easily by lifting the disk and checking for pitting, scoring, wire drawing, and erosion.

Valves with damaged or pitted seats and disks will not seat properly and must be removed from the line for regrinding or replacement of the seats and disks. On spring-loaded check valves the spring should be checked for twisting, cracking and metal fatigue. Weak or damaged springs must be replaced. All threaded surfaces must be inspected for damaged, galled, or stripped threads.

Check Valve Repair Methods

Most check vales have replaceable seats, disks, plugs and caps. Special tools are available from valve manufacturers for valve seat removal. The use of improper tools, when removing the valve seat, may distort the valve body and ruin the valve completely. Before any repair work is done, each valve must be thoroughly washed in cleaning solvent and blown dry with clean compressed air. All gaskets found on the valve flanges or caps should be discarded. The use of old gaskets may result in serious leaks after the valve is put back on the line.

Valve disks are removed by unscrewing the plugs on both sides of the valve and lifting out the disk from the valve body. Metallic disks should be inspected for pitting, etching, scoring, wire-drawing and cracking, and all defective disks should be replaced. Non-metallic disks should be checked for wear, distortion, swelling, and fraying.

The valve seat should be inspected in the same manner as the valve disk. If the pitting and scoring is slight, the valve seat can be repaired by regrinding. However, if the damage is extensive or if the seat is cracked, cocked or extensively worn, it must be replaced. It is exceedingly important to use manufacturer's recommended tool for the valve seat removal. The tool must fit snugly in the provided lugs on the valve seat body and must be held straight during the removal procedure. After the valve seat is unscrewed from the valve body, the mounting threads in the valve body should be thoroughly cleaned with a stiff brush and cleaning solvent until all foreign matter is removed, then blown dry with air.

Valve plugs should be cleaned with a stiff wire brush and cleaning solvent, and inspected for damaged, corroded, stripped, or galled threads. All defective plugs must be replaced.

If the regrinding of the seat and disk is necessary, the valve should be mounted in a suitable vise, making sure that it is as level as possible. A moderate amount of grinding compound should be used. Some check valves can be ground without removing them from the line, but a much better job can be done if the valve is overhauled in the shop. When grinding the seat, the disk should be pressed with a slight pressure against the valve seat, using a screwdriver or other suitable tool. The disk should be rotated back and forth at about a one-quarter turn and then raised and rotated to a new position. Continuous rotation of the valve disk all the way around should be avoided, since this may cut grooves in the valve seat. Generally, it will take from 3 to 5 min. to grind the valve seat.

To check for proper valve seating, all traces of grinding compound should be removed from the valve seat and disk, and both seating surfaces thoroughly cleaned. Using a chalk or pencil, lines should be drawn across the valve seat face, spacing them about 1/8 in. apart. Then the disk should be pressed against the valve seat and rotated a few times. If the lines are rubbed off, the grinding job is completed and the valve will seat properly. If some of the lines are not rubbed off, the grinding should be repeated.

After grinding the check valve, the entire valve should be thoroughly cleaned, making sure that all traces of the compound are removed.

When reassembling the valve, new gaskets should be used. The valve cap bolts and nuts should be pulled up evenly, on diametrically-opposite positions, to prevent warpage of the valve cap. All valve plugs must be tight and the valve disk should swing freely on its pivot. After the valve is reassembled, all openings should be covered with a clean cloth or suitable plugs to prevent entrance of dirt and foreign matter.

The check valve, the real work-horse of the valve family, is known for its dependability and performance. If properly installed, it will require practically no maintenance and will outlast all other valves in the piping system. While simple, it is vital to power piping.