AKRAMANUJAN.ORG

22ND JANUARY 2019

Boilers continued (Function 4 Orals)

5. BOILER MOUNTINGS

The boiler mountings are the attached valves and cocks directly on the shell plating. Since they are attached to the shell plating through suitable holes cut on the plating, the strength lost due to the hole is suitably compensated by doubler pads welded around the holes. The location of the mountings is shown in the sketch given in (fig 23). The mountings are the same for all boilers and their locations as well as their purpose is given as follows:

Safety valves :- These are two spring loaded valves mounted on the same chest with one opening, to relieve the boiler of excess pressure. These valves are provided with manually operated easing gear to be operated in certain emergency situations. Its location is on the steam side of the boiler at the highest location.

Main stop valve:- This is the main steam outlet valve from the boiler leading to the various users. Location on steam side of vessel.

Air vent :- This valve is the outlet to remove air from the pressure shell during warming up of the boiler and also used to remove air during hydraulic testing of the boiler. Location steam side of vessel at the highest point.

Pressure gauge cock:- This is the attaching cock to the pressure gauge to monitor the steam pressure continuously. Location steam side of vessel.

Gauge glasses :- These are two in number fitted directly on the pressure vessel at the inter-face between steam and water. It is continuously monitoring the safe working level in the boiler.

Main feed check valves :- Set of two valves to control and admit feed water into the boiler. Location water side of the pressure vessel.

Auxilliary feed check valves :- Exactly identical to the main feed check valves. To be used only if main feed line or main feed check valves are defective. Hence it is a stand-by connection. Location same as main check valves.

The main feed check valves and main feed line is always in use, except when they are defective, in which case the auxiliary feed line and auxiliary feed check valves come into operation. The feed heater and feed controller are connected to the main feed line and these accessories are not available when operating on auxiliary feed line.

Scum blow down valve :- This valve is used for removing scum collecting on the surface of the water as the name suggests. It is frequently used to give flash blow downs to control and maintain the density of boiler water. Location

water side of pressure vessel.

Main blow down valve :- To empty the entire water contents of the boiler for cleaning, inspection and repairs when needed. Location lowest point on water side of pressure vessel or boiler.

Salinometer or sampling cock:- Provided to take out sample of boiler water for testing purposes. Location water side of pressure vessel.

Apart from the above ten mountings boilers operating with forced circulation exhaust gas economisers are provided with two more mountings, the steam inlet valve entering the boiler steam side and the water outlet valve to circulating pump located on the water side. In the sketch shown below, a coaming about 300mm high is provided . This is a water tight enclosure and is provided as a fire protection devise to contain any risk of fire by oil spillage around the boiler. The 45 litre semi portable chemical foam extinguisher when discharged into this space will fill up foam to a height of 150 mm within the coaming . The enclosed coaming area is calculated on this basis. The area within the coaming , is therefore maintained dry and hence the need for the funnel scupper to drain the water from safety valve and gauge glass drains to the bilges below.

LOCATION OF BOILER MOUNTINGS (FIG 23)

SAFETY VALVES :- At least two safety valves have to be fitted to any one boiler. They may both be fitted on the same chest with one connection to the boiler pressure vessel..

The safety valves must never be less than 38mm in diameter and the area of the valve can be calculated from the following design formula:

C*A*P=9.81*H*E

Where H= Total heating surface area in m2

E = Evaporation rate in kg of steam per m2 of heating Surface per hour.

P= working pressure of boiler in MN/m2 absolute.

A = Aggregate area through the seating of the valves in Mm2

C is a discharge coefficient whose value depends on the type of valve. C= 4.8 for ordinary spring loaded valves.

C = 7.2 for high lift spring loaded safety valves. C = 9.6 for improved high lift safety valves.

C = 19.2 for full lift safety valves.

C = 30 for full bore relay operated safety valves.

The most commonly used safety valves on marine boilers are the High Lift and Improved High Lift safety valves. Both these valves are almost identical, except for the following differences:

The ordinary high lift safety valve has winged valves to guide the valve, whereas the valves in the improved high lift safety valves are wingless, the guidance given by a ported guide plate.(FIG 24)

In the improved high lift safety valve a floating cup and a plate piston is provided to give additional force.(FIG 25)

These differences are shown in the two sketches given below.

Dismantling and overhauling the valves:- Safety valves are regularly overhauled and set during survey of the boiler, which occurs every two years

To overhaul the valve, the boiler is to be in cold and isolated condition.

Method is as follows:

The easing gear cable is disconnected from the easing gear.

The easing gear is dismantled by opening the lever bearing keeps.

The cotters connecting the caps to the spindles are removed after opening their locking devices and the caps taken out.

Note position of both compression nuts by counting the number of exposed threads and noting the same for future reference when assembling the valves .

Unscrew both compression nuts to relieve the springs of their compression.

Dismantle the top cover along with compression nuts.

Dismantle the Bonnet holding nuts and take out the bonnets along with spindles, springs, spring plates and attached valves. The valves can be dismantled from the spindles . the valves are in complete dismantled condition for inspection.

Inspection of parts:- The inspection of the parts and suitable corrective action is as follows:

The spindle should be checked for any bends and this can be verified exactly

by centering and truing on the lathe.

The springs should be checked for any deformity or permanent set and this can be done by comparing their heights and also comparing with the spare spring. Springs should be renewed if they are deformed or permanently set.

The compression nut should be tried on the cover and checked for radial and axial play. If the nut is slack it should be renewed.

Check the valve and valve seat for any cuts or ridges. Normally such defects do not occur since the valves very rarely open due to excessive pressure The valve requires light lapping using jewellers rouge

Ensure the drain hole and connecting drain pipe to the funnel scupper are both clear.

The inspection being completed and suitable corrective action taken, the valves can be assembled after survey and compression nuts tightened to the original settings, The caps and easing gear will be assembled after he valves are floated(set) by the surveyor.Comparison of various types of safety valves :- The types of safety valves

fitted on boilers are basically two types:

spring loaded types which are the ordinary high lift safety valve, the improved high lift safety valve and the full bore safety valve.

Fixed weight loaded safety valves, named the dead weight safety valve and the lever safety valve.

The deadweight safety valve and the lever safety valve are not fitted on marine low pressure boilers although they are more efficient than spring loaded valves because they are capable of giving a sustained lift which the spring loaded valves cannot give because of the inherent quality of a spring to increase the resisting force when it is compressed and this increase in resisting force makes the spring to shut the valve. Hence the valve will continuously chatter or feather not allowing a sustained lift. Whereas in a deadweight safety valve or the lever safety valve the weight component is constant and hence resisting force is constant giving a sustained lift. The reason why the deadweight or lever safety valve is not fitted on marine boilers is that due to the rolling and pitching of the ship in rough seas the weight component changes causing the valve to lift at lower pressures and hence cannot be fitted on marine boilers.

To give sustained lift the valve is designed with a lip to give sustained lift which works as follows.(FIG 26)

FIG 26 Referring to the figure above if P IS the set pressure acting under the valve and

P1 is the reduced pressure acting under the lip, then; The set resisting force of the spring is given by

D2/4* p , and the additional force required to give sustained lift to keep the valve open is given by

P1 * A and hence the valve gets sustained lift.

The three spring loaded safety valves ,ordinary high lift and improved high lift, all make use of the specially shaped valve as explained above.

The area of the valve chest connecting to the boiler must be at least equal to one half of the aggregate area of the valve cross section or inlet passage to the valve. The waste steam pipe and steam and steam pressure from the valves must have a cross sectional area of at least :

* aggregate area of valves for ordinary, high lift, improved high lift safety valves.

2* aggregate area of valves for full lift safety valves

3* aggregate area of valves for fill bore relay operated valves.

Typical values of lifts are as follows:

Where C= 4.8 -------------lift=D/24 (ordinary type) C= 7.2 AND 9.6 -----lift=D/12.

C = 19.2 and 30 ----lift= D/4

For low pressure boilers the improved high lift safety valve is generally employed.

The above design features are all made to reduce the phenomenon of accumulation of pressure which is the manifestation of the additional force required to give sustained lift. The accumulation of pressure is the difference in the pressure read on the gauge and the set pressure. The set pressure is also termed the working pressure or design pressure and normally stamped on the name plate on the boiler.

Accumulation of pressure test:- Classification societies require that, when initially fitted to boilers, safety valves must be subjected to an accumulation of pressure test to ensure the valves are of the correct discharge capacity for the boiler. To conduct such a test, all feed inlets and steam outlets to and from the boiler respectively, must be closed and maximum firing rate arranged for. Accumulation of pressure must then not exceed 10 percent of the working pressure. Duration of the test (water permitting) is not to exceed 15 minutes for smoke tube boilers and 7 minutes for water tube boilers. In the case of water tube boilers the test may be waived if damage to superheaters or economisers could result from this test.

Procedure for setting the valves :- After every survey the safety valves have to be set by the classification society surveyor for all auxiliary and composite boilers. For forced circulation exhaust gas economisers and independent exhaust gas boilers the ships Chief engineer is authorised to set the safety valves. This is an operation convenience because the surveyor cannot be on the ship at sea where the separate exhaust gas boiler or the forced circulation exhaust gas economiser safety valves will be set. The procedure to set the safety valves is given below after the boiler has been properly warmed up and the initial pressure is about 1kg/ cm2.

During the setting procedure the accumulation of pressure is also verified but not for the stipulated time. The valve is set and firing is prolonged whilst the surveyor goes out to check the flow of steam from waste steam pipe and comes back to see the increase in set pressure which normally is about 0.25 kg/cm2.

As per the rules the error in setting is permitted up to 3% of working pressure.

BOILER SAFETY VALVE SETTING PROCEDURE:

1) A Gagging tool is used alternatively on the valve to prevent simultaneous lifting of the valves at the set pressure.(FIG 27)

2) Use the standard boiler gauge brought by the surveyor if the boiler pressure gauge is not caliberated.

3) Remove the caps keep the valve spindle of one valve (port) gagged. The

other valve will be set first (stbd).

4) Keep the compression nut screwed down, on the valve to be set, till the previous marks.

5) Raise steam pressure slowly and bring pressure to set pressure.

6) A slight hissing sound will be heard. This is the floating condition of the valve. In this condition the valve will sit on its seat by slight pressure on the spindle with the palm of the hand and will not come back unless tapped.

7) Reduce pressure, remove gagging tool and fit out the other valve. Repeat

above operation on this valve for setting.

8) When both valves are set remove gagging tool. Fire the boiler at maximum capacity. Note full flow of waste steam from waste steam pipe. Note pressure on gauge. The pressure should not exceed 10% of the set pressure. On satisfactory completion of setting, refit caps, cotters, locks and easing gear. Test easing gear and following safety alarms and cut outs: -

(a) H.P cut off and alarm

(b) Low water alarm and cut off

(c) High water alarm and cut off

A good Safety Valve lifts smartly at its adjusted pressure and, after it has relieved excess pressure, shuts with equal smartness.

The ability of a valve to lift smartly and fully, without feather, is a matter of design; in all high capacity types it is achieved by allowing the steam from the initial lift to impinge on additional lifting surfaces, either in the form of a lip or piston, in a guiding cylinder.

Valves which embody the foregoing characteristics, other than the Cockburn Improved High Lift, are as Follows: Full bore, Hylif, consolidated and similar types. Full Bore Safety Valves

These safety valves have four times the discharge capacity of an ordinary

spring-loaded valve, their arrangement is shown in the following figure (Fig 28)

It will be noted from the drawing that each main valve is operated by its own control valve, and that both of these valves are in direct communication with the steam drum or super heater header on which they are mounted.

The action of the valve is as follows: Steam from the boiler exerts pressure on the underside of the control valve, which lifts at a predetermined pressure. In lifting, the top part of the control-valve lid blanks off ports leading to atmosphere and allows steam to pass through a passage leading to the piston on the main valve, spindle causing the main valve to open against the boiler pressure (the area of the piston is about twice that of the valve.)

When the boiler pressure drops the control valve shuts down, and in so doing uncovers the ports above it which lead to atmosphere. This relieves the pressure from the main valve piston, and the boiler pressure then shuts the main valve. The foregoing is the action of one control valve and its main valve; there are, of course, always two or more control and main valve units per boiler.

In view of the high temperatures at which the working parts of these valves have to work, it has been found advantageous in some high superheat installations to fit the control valves on the saturated steam drum separate from the main valves, and to pipe the saturated steam from these valves to operate the main valves on the super heater header.

The Hopkinson Hylif safety value

These valves, incorporating a full-lift feature, are designed for working pressures of up to 62 bar, the arrangement being as shown in Figure . When steam pressure rises to the set pressure, the valve discharges with a small lift on the principle of the ordinary safety valve. This initial opening allows the escaping steam to exert its pressure over the full area of the bottom of the valve and increases (Fig 29 & 30)

The lift until the bottom face of the valve has entered the valve guide; at this point the escaping steam is deflected downwards by the bottom edge of the guide, and the consequent reaction pressure lifts the valve to its full-open position (see detail). At this final stage of valve lift the discharge area between the seat and the valve is claimed to be equal to the net area through the seat throat, and the discharge capacity is at its maximum.

When the discharge pressure has been relieved the valve begins to close, as it emerges from the valve guide the reaction pressure ceases and the valve shuts down cleanly without simmer

Materials of construction for ordinary high-lift and improved high-lift.

Safety valves.:- Improved high-lift safety valves are usually fitted on moderately higher pressure boilers fitted with super heaters and ordinary high- lift safety valves are fitted on comparatively lower pressure boilers using saturated steam. The table given below shows the difference in materials used Table 1

Except for the chests, bonnets, covers and springs mild steel for all other components is not used to avoid corrosion of the components. Cup and piston are not used in ordinary safety valve.

Difference between safety valve and relief valve:- Both the words mean the same since their objectives are the same and that is to relieve excess pressure. Historically the boiler is an invention over 150 years old and when it was put into industrial use there was a need to provide a means of safety to prevent any explosion to the boiler which could cause fatality and hence this valve which relieves the boiler of excess pressure was termed a safety valve as the word suggests safety and prevention of loss of life. With the progress in industrialization and development of different machinery employing similar pressure relieving devices, the word relief valve came into usage for all other machinery where a pressure build up is possible.

There is however a difference between the boiler safety valve and relief valves used elsewhere and this difference is subtle and explained as follows

In a boiler the source of energy is the fuel and its burning is primarily independent of the steam generation, taking into consideration that any interdependent safety device provided, can fail at times, and in which case, the boiler must get relieved of the steam pressure as it is being produced so long as the fire is on. Hence the need for sustained lift and an extra provision of the manual easing gear.

Consider the case of a diesel engine cylinder head fitted with a relief valve. This relief valve will lift if there is an increase in pressure caused by a stuck exhaust valve or excessive pressure caused by starting air being admitted while the engine is working. These defects will cause only a momentary increase in pressure as the piston approaches the T.D.C.. Hence there is no need for sustained lift. This is the basic difference between the boiler safety valve and a relief valve as fitted on the diesel engine cylinder head. Consider the main air bottle relief valve. This valve comes into operation when the air compressor is not stopped when the pressure inside the air bottle is more than the set value, The air compressor is a mechanical device and the pressure build up within the bottle has to be overcome by the air compressor and when this happens the valve will lift and relieve the excess pressure. The build up of pressure is also momentary since the compressor is working like the diesel engine during its compression stroke and pressure build up is momentary and hence there is no need of sustained lift.

Hence relief valves need not be fitted with manual easing gears. Relief valves can be removed tested and set by a hydraulic test pump, whereas the boiler safety valves are to be set in the insitu condition with boiler under pressure.

Oral questions on safety valves:-

(1) What is the difference between a spring loaded safety valve and a dead weight safety valve or a lever safety valve.

(2) State the sustained lifts of ordinary, improved and full bore safety valves. How are they provided for.

(3) What is accumulation of pressure and what is its measure.

(4) What is the full available lift in a safety valve.

(5) Describe the procedure of setting the safety valves after completion of the boiler survey.

(6) 2What is the difference in procedure of setting the valves of the auxiliary boiler and the exhaust gas boiler.

(7) What is the difference between safety valves of the boilers and relief valves provided elsewhere.