reseating machine. With a fine file, remove all high spots on the surface of the flange upon which the chuck jaws must fit. Note that a valve reseating machine can be used ONLY with a valve in which the inside of the bonnet flange is bored true with the valve seat. If this condition does not exist, the valve must be reseated in a lathe and the inside flange bored true.
Before placing the chuck in the valve opening, open the jaws of the chuck wide enough to rest on the flange of the opening. Tighten the jaws lightly, so the chuck grips the sides of the valve opening securely. Tap the chuck down with a wooden mallet until the jaws rest on the flange firmly and squarely. Then tighten the jaws further.
Adjust and lock the machine spindle in the cutting position and begin cutting by turning the crank slowly. Feed the cutter slowly, so very light shavings are taken. After some experience, you can tell whether or not the tool is cutting evenly all around. Remove the chuck to see if enough metal has been removed.
Be sure the seat is perfect. Then remove the 45-degree cutter and face off the top part of the seat with a flat cutter. Dress the seat down to the proper dimensions as follows:
|Width of Seat||Size of Valve|
|l/16 inch||l/4 to 1 inch|
|3/32 inch||1 l/4 to 2 inches|
|l/8 inch||2 l/2 to 4 inches|
|3/16 inch||4 l/2 to 6 inches|
After the refacing, grind-in the seat and disk. Spot-in as necessary to check the work. A rough method of spotting-in is to place pencil marks at intervals of about l/2 inch on the bearing surface of the seat or disk. Then place the disk on the seat and rotate the disk about a quarter of a turn. If the pencil marks in the seating area rub off, the seating is satisfactory.
When the stem of a globe valve is in good condition, stuffing box leaks can usually be stopped by setting up on the gland. If this does not stop the leakage, repack the stuffing box. The gland must not be set up or packed so tightly that the stem binds. If the leak persists, a bent or scored valve stem may be the cause of the trouble.
Coils (string) and rings are the common forms of packing used in valves. The form to be used in a particular valve is determined, in part, by the size of the packing requi red. In general, rings are used in valves that require packing larger than 1/4 inch. When a smaller size is required, string packing is used.
When you repack a valve stuffing box, place successive turns of the packing material around the valve stem. When string packing is used, coil it around the valve stem. Bevel off the ends to make a smooth seating for the bottom of the gland. Then put on the gland and set it up by tightening the bonnet nut or the gland bolts and nuts. To prevent the string packing from folding back when the gland is tightened, wind the packing in the direction in which the gland nut is to be turned. Usually, where successive rings are used, the gaps in the different rings should be staggered.
Gate, globe, angle, and stop-check valves are made to back seat the stem against the valve bonnet when the valve is fully opened. Back seating of these valves is a safety feature to eliminate the stem being forced out under pressure while the valve is fully opened. Back seating makes repacking of the stem stuffing box possible under pressure; however, you should attempt this only in emergencies and with extreme caution.
Q1. What structure of a valve distinguishes one type of valve from another?
Q2. Direction of flow is indicated on the exterior of a check valve in what manner?
Q3. Hydraulic control valves are used in what type of plumbing system?
Q4. To reface valves, you can use the valve reseating machine only when what condition exists?
LEARNING OBJECTIVE: Recognize the different types of valve accessories and the methods used to maintain them.
Accessories that aid in the control of valves include valve boxes, floor stands, and post indicators. Each of these supporting materials is discussed briefly below.
Street valves must be accessible for turning them off and on; and, with large valves, the entire valve should be accessible for servicing. Since valves are placed at various depths, valve boxes are made in twoContinue Reading