Swing check valves use a disk that is attached to the valve body by a pinned hinge and is closed by gravity during a no-flow condition. This type valve is sometimes designed with a spring to assist closing the valve. Pressure caused by flow forces the hinged disk up to open the valve. But, pressure in the opposite direction will force the hinged disk back on its seat to close the valve. The proper positioning of the valve, with refer- ence to the horizontal, is very important to ensure proper check valve operation. Since the downward force of gravity is necessary for proper operation, a check valve installed upside down or at any angle other than horizontal may not function as intended. Also, since this valve allows flow in only one direc- tion, it must be installed correctly. Most check valves will have a flow direction arrow on the body. If no arrow is visible, the inlet side of the valve will be the side with the hinge pin. VALVE MAINTENANCE All valves require proper care and maintenance, as does other more complex equipment, to ensure they are kept in optimum working order. The principle difficulties encountered with valves are leakage past the seat and disk, leakage at the stuffing box, sticking valve stems, and loose valve disks. Losses due to leakage that is not corrected mount up considerably over time. For example, over a period of a month, a small 1/32 inch hole would waste 69.552 cubic feet of air at 100 psi, 3,175 pounds of steam at 100 psi, or 4,800 gallons of fuel at 40 psi. The ABF should know how to prevent and correct these faults. Valve Leakage Causes and Remedies Valve leakage, generally caused by failure of the disk and the seat to make close contact, may result from any of the following: —Foreign substances, such as scale, dirt, or heavy grease lodged on the valve seat may prevent the disk from being properly seated. If the obstructing material cannot be blown through, the valve has to be opened and cleaned. —Scoring of the valve seat or disk, which may be caused by erosion or by attempts to close the valve on dirt or scale, results in leakage. If the damage is mini- mal, the valve may be restored to proper working order by grinding. If the damage is more extensive, the valve must be reseated and then ground. —A warped disk may result if the guides fit too tightly, if the spindle guide is bent, or if the valve stem is bent. Using a valve disk or body that is too weak for the purpose for which it is used permits distortion of the disk or seat under pressure. If this occurs, replace the valve. Packing Gland Leakage Packing gland leaks can be remedied by tighten- ing the gland or repacking it. But, the gland must not be tightened nor packed so tightly that the stem binds. If the leaks persist after either or both of the remedies are applied, a bent or scored valve stem may be the cause. Packing for the valve may be either of the string type or of the ring type. String packing is ordinarily used for small valves in low-pressure systems. Ring packing is used for large valves and for all high pres- sure valves. When replacing the packing on any type of valve, be sure to use the correct size and type. The packing must be large enough to fill the space be- tween the valve stem and the packing box. It also must be made of material that is suitable for the pressure and temperature to which it will be exposed. To pack a valve with string packing, place succes- sive turns of packing in the space around the rod. Bevel off the ends of the packing to make a smooth fit and tighten the packing gland nut or the bonnet nut to compress the packing. String packing should always be wound in the same direction as the gland nut is to be tightened so tightening the nut does not cause the packing to fold back upon itself. To pack a valve with ring packing, cut the ends of the rings square and even so they make a level butt joint. Be sure to stagger the joints in successive rings. In some gate, globe, and one-way check valves, the packing gland may be repacked under pressure, when necessary. These valves are constructed with the stem back-seated against the bonnet when the valve is wide open. High-pressure valves are provided with a pressure leak-off connection. The pressure leak-off connection is sealed to the outside with a pipe plug. Extreme care should be taken to see that the valve is firmly back-seated before the plug is re- moved. Normally, repacking valves under pressure is NOT done by an ABF. If a valve must be re- packed under pressure, ensure ALL SAFETY PRECAUTIONS are followed. 4-22