INTRODUCTION
Boiler safety valves protect the boiler from overpressurization. Each boiler is fitted with safety valves to allow steam to escape from the boiler when pressure rises above specified limits. Steam escapement piping directs the escaping steam to the atmosphere.
REFERENCES
(a) Main Boilers Repairs and Overhaul Manual 59221-01-010
(b) Boilers NSTM Chapter 221
(c) Principles of Naval Engineering NAVEDTRA 10788 series
(d) Boiler Technician 1 & C NAVEDTRA 10536 series
(e) Boiler Technician 3 & 2 NAVEDTRA 10535 series
(f) Remote Operated Safety Valve System for 1200/600 PSI Boilers S6435-QG-MMO-010
INFORMATION
A. Each boiler is fitted with safety valves which allow steam to escape from the boiler when pressure rises above specified limits. The capacity of the safety valves installed on a boiler is great enough to reduce boiler pressure to a safe point when the boiler is being operated at its maximum firing rate with all steam stop valves completely closed. A flow of steam through the superheater must be maintained to prevent overheating; A safety valve on the superheater outlet will lift or be actuated at the same time the first steam drum safety valve lifts. This ensures steam flow through the superheater.
B. Associated terms
1. Lift or pop refers to the opening of the safety valve.
2. Reseat refers to when the valve closes tightly. It should do so without chattering. The reseating of a safety valve is required to be 3-6% below actual lift pressure.
3. Blowdown is the duration of time between a safety valve lifting and reseating.
4. Simmering is steam that leaks by the disc and seat before the safety valve lifts. This is an acceptable condition within 1% of lift pressure.
5. Chattering is the rapid lifting and reseating of the valve disc.
C. Non GIS operated safety valve systems consist of three boiler steam drum safety valves located forward to aft along the top center of the steam drum and identified from front to rear as NR2 drum, NR1 drum, and the pilot actuating safety valve. To ensure a flow through the superheater, another safety valve is located on the superheater outlet piping prior to the main steam stop valve.
D. There is a functional difference between boiler safety valves and ordinary relief valves. The amount of pressure required to lift a relief valve increases as the valve lifts, since the resistance of the spring increases in proportion to the amount of compression. Therefore a relief valve opens slightly at a specified pressure, discharges a small amount of steam, and closes at a pressure which is very close to the pressure that causes it to open. Boiler safety valves are designed to open completely at a specified pressure and reseat at a significantly lower pressure.
E. Different types of safety valves are used on naval boilers. Nozzle reaction safety valves are the most common, but some older ships have huddling chamber safety valves. The design and operation of these valves are basically the same. Both open completely, or pop, when a specified pressure is reached and remain open until a specified pressure drop or blowdown has occurred and the valve reseats. Safety valves must close tightly, without chattering, and must remain tightly closed after seating.
F. Safety valves are constructed much like a relief valve except for the addition of a nozzle ring and guide ring which assist in the lifting and reseating process, a disc and disc insert which form a leak-proof seal when the valve is closed, and a disc guide which ensures an even vertical rise of the disc when it lifts. The guide ring and nozzle ring set screws prevent the rings' settings from changing through use and vibration. The bonnet is bolted to the body and houses the spindle and the spring, which act together to keep the disc seated. Also on the top of the bonnet is the adjusting bolt and lock nut which provides the means of setting and locking the spring force. The cap assembly and lifting fork (lever) provide security for the adjusting bolt and lock nut and provides an attachment point for the hand easing gear. This construction is the same for most safety valves. The pilot actuating safety valve is slightly smaller and has an orifice on the outlet to create a slight back pressure through an actuating line to the superheater safety valve. This back pressure will cause the superheater safety valve to lift simultaneously with the pilot valve, thus ensuring flow through the superheater. Where simple spring-loaded superheater safety valves are used, they are practically identical in construction to drum valves, except that special materials are used to withstand the higher temperatures.
G. The opening of a safety valve is produced by two stages of reaction, working together to produce a continuous pop. First, the escaping steam strikes the nozzle ring and changes direction. The resulting reactive force pushes the disc up, giving the pop action. After the valve has popped to a moderate lift, the escaping steam begins to react against the guide ring and the reaction of the deflected steam pushes upward against the underside of the disc, lifting it still higher. In this way, the valve attains a full lift. The initial lifting force at the opening is affected by the position of the nozzle ring in relation to the valve seat. The final lifting force, which brings the valve to full lift, is controlled by the position of the guide ring. With decreasing pressure, the disc and holder will drop closer to the seat and will then snap to the closed position as the reactive lifting force is lost. The guide ring is the principal blowdown control ring. Turning the guide ring to the right raises it and shortens the blowdown, turning the guide ring to the left lowers it and lengthens the blowdown. The nozzle ring principally controls the sharpness of the valves opening and closing action. The nozzle ring, if properly set upon first installation, should require no further adjustment while in service. However, if proper functioning of the valve cannot be obtained by movement of the guide ring, resetting the nozzle ring may be necessary.
H. During operation, the pilot valve is the first safety valve to lift and the last to reseat. It actuates the superheater safety simultaneously. The superheater safety valve is also set to lift at the same or slightly lower pressure than NR1 drum safety valve if the pilot fails to actuate the valve. If pressure continues to rise the NR2 drum safety valve will also lift at a specified pressure.
I. During normal operation safety valves should strictly be left alone as long as their operation is satisfactory and all requirements are met. The settings of safety valves for all ships are determined by Naval Sea Systems Command (NAVSEA) and cannot be altered without NAVSEA approval. If safety valves cannot be properly set, the boiler shall be placed out of commission until the safety valves are repaired.
J. A hand easing gear is attached by cables to the safety valves and is used to manually lift the boiler safeties remotely during casualty situations. Prior to every boiler light-off the hand easing gear must be inspected to ensure the cables are attached to the lifting arms and are free to move in accordance with the Engineering Operational Sequencing System (EOSS) Master Light Off Checklist (MLOC).
K. Visual checks of a safety valve can be conducted quickly and easily. The fork lever clearance is usually 1/16" and can be verified by referring to the applicable manufacturer's technical manual. Leakage from the flanges or through the valve seat is not allowable under wet lay-up. Any signs of tampering with the locks and seals must be immediately reported to the Engineer Officer or Engineering Duty Officer. Ensure the springs of the valve are not painted as this will effect their operation. The valve should be kept in good general repair and relatively free from corrosion.
L. Safety valve leakage can be caused by sediment or chemical residue between the disc and seat or the misalignment of flanges or escapement piping. In addition, the hand easing gear may be restricted from expanding with the boiler due to an obstruction or interference causing a strain on the valve/valves. Sediment or chemical residue may be removed by lifting the safety valve with steam. One of the most common problems encountered with spring loaded safety valves is leakage past the seating surfaces. Because of the hard metallic seating surfaces and the relatively low seating forces, some leakage may be present at full boiler pressure. As valves are degraded by time and numerous pops, the leakage rate of the valve will increase. When valves have been degraded by wear and leakage becomes excessive, seating surfaces should be examined when possible and remedial action taken as necessary. In general, safety valve leakage, although a nuisance, is not grounds for curtailing boiler operation. At present, the only criterion available for determining whether safety valve leakage has reached a point requiring repair action is evidence of a steady plume of steam (three feet long in calm air) emitting from the exhaust line at the stack. New or reworked valves are not allowed to have any leakage.
M. To steam test or set safety valves, there must be some means of keeping safety valves closed which are set at a lower pressure. This is also necessary when hydrostatically testing the boiler at pressures higher than safety valve settings. To accomplish this, gags are used. Similar in appearance to a gear puller or C-clamp, gags are used to apply a small amount of pressure to the stem of the valve, holding the disc onto the seat. Gags shall not be placed on safety valves when the boiler is cold except before a hydrostatic test. Thermal expansion of the safety valve spindle as the boiler temperature rises may result in severe over-gagging stress on the spindle, possibly causing damage to the spindle if the gags are applied when the boiler and valves are cold. Gags shall not be placed on a steaming boiler during warmup status until the boiler pressure is 100 to 200 psi below the lowest safety valve setting or 75 percent of the working pressure, whichever is higher. Also ensure that the gags and valve temperatures have equalized before tightening the gag finger tight.
1. Prior to setting safety valves, the steam gage being used to establish set points should be checked for proper calibration. The hand easing gear cable assemblies and the cap assembly must be removed from the valves to allow installation of the gags.
2. Refer to the PMS for specific procedures for setting safety valves. Each individual valve of each boiler shall be lifted by steam and reset if at variance with the authorized popping and reseating pressures. Between pops, drop the pressure to operating pressure.
3. Some boiler designs are peculiar in that the pilot operated spring loaded superheater safety valve and the lower drum safety valve have similar lift settings. To eliminate possible confusion when these valves are tested independently (and lifted after gag removal), follow the maintenance requirement card (MRC). The following summarizes the procedure:
4. Gag all valves except the superheater safety valve.
5. Set the superheater valve, preferably to minus tolerance or lower than the setting of NR1 drum safety. Because the temper of the superheater valve spring is affected by temperature, complete cool-down of the valve between blowdowns (at least 1 hour) is necessary. With increasing temperature, valve blowdown tends to decrease. For a final approval test, the superheater valve shall be popped only in conjunction with the pilot valve.
6. Gag the superheater valve carefully and test the drum valves in succession (in order of descending set pressure).
7. Set the drum valve with the lowest set pressure, preferably to plus tolerance.
8. Remove the gag from the drum pilot valve.
9. Test and set the pilot valve.
10. Remove the gag from the superheater valve.
11. Test pop the pilot and superheater valves in combination.
12. Enter the test results in the Engineering Log, boiler water treatment log, fireroom operating log, and on the MRC.
N. Safety valves shall be tested by steam under the following circumstances:
1. In accordance with PMS
2. After the valve has been gagged for any reason
3. When valves have been removed, reworked or tampered with
4. If accidentally lifted by water during a hydrostatic test
SAMPLE 600 PSI SAFETY VALVE SETTINGS
LIFT RESEAT
Drum pilot 750+/-5psi 3-6% of lift pressure
Superheater 765+/-5psi 3-6% of lift pressure
NR1 Drum 765+/-10psi 3-6% of lift pressure
NR2 Drum 775+/-10psi 3-6% of lift pressure
O. The remote pilot-operated superheater safety valve system consists of a remote hydraulic pilot panel assembly, the superheater outlet safety valve, and NR1 and NR2 drum safety valves. This system replaces the pilot actuating safety and the superheater safety valve of the conventional spring type system previously covered. This system has several advantages over the simple spring loaded valves. It eliminates the pilot actuating valve with the pilot panel assembly. This reduces the chances of safety valve leakage and eliminates the setting or adjusting another valve on top of the steam drum. This system is installed on 600 psi steam ships and has proven to be very reliable with minimal failures.
P. The pilot valve is assembled on a heavy metal backing plate and enclosed in a cabinet which is mounted in a convenient location in the fireroom. Unlike a conventional steam pilot actuating safety valve, it does not function as a safety valve since it does not exhaust steam. The pilot valve unit assembly senses boiler steam pressure and uses feedwater pressure to provide the hydraulics to control all functions (pop/reset) of the superheater safety valve when the three-way valve is in the automatic position. The three-way valve is located inside the pilot valve panel and can be manually operated to by-pass the pilot valve unit (to open the superheater safety valve by using the hand easing gear). The filter canister, located in the back of the pilot valve panel, keeps steam and dirt out of the system and provides a reservoir of cool, pressurized boiler water for proper valve operation
Q. The superheater safety has a nozzle and disc to contain boiler pressure, while a spring assists in holding the disc closed at low boiler pressure. The piston transmits the hydraulic force to the stem, keeping the disc closed. The gag stem can be used to hold the disc closed regardless of the steam pressure. A packing gland seals the area around the gag stem to prevent leakage. The purge valve is installed to vent air from the valve after boiler repairs which require the boiler to be placed on a dry lay-up or drained of boiler water.
R. When setting safety valves, no external gag is needed for the superheater safety and pilot assembly. The superheater safety valve and pilot assembly are kept from lifting by installing a gag on the pilot pressure sensing steam which is located in the control panel This gag is lowered into position onto the pilot valve only when needed. It gags the pilot assembly only, not the superheater safety valve. This keeps the superheater safety valve from lifting. If the pilot panel has been removed or is inoperable, there is an additional gag for the superheater safety that may be used
S. Note: No cool-down period is needed on this system's superheater safety valve unless the valve is hot to the touch. Proper blowdown/venting of the pilot valve assembly must be accomplished prior to lighting off the boiler.