Boiler is a subtle, yet a necessary machinery onboard ship which is actually a very high-pressure vessel operative between high pressure ranges depending on the sort of ship and its necessities, like for steam ships or LNG vessels the boiler pressure ranges around 60 bars and even more* than that since those ships are steam propelled. Coming to tanker vessels the boiler pressure ranges around 16-30 bars*, and in case of dry cargo ships like container vessels, bulk carriers and other general cargo vessels the range is around 6-15 bars*. Now for a safe and sound operation of these we need to fit safety valves safety valves in boilers and relief valves while its manufacturing and get it approved by the Classification Society subjected to Accumulation pressure test which ensures that the valves are of correct discharge capacity of the boiler. The Classification Societies demand that at least two safety valves are fitted to every boiler, however in practice it is usual to suit three valves i.e., two are supposed to be on the steam drum, and another on the super heater outlet header.
Table of Contents
Types of safety valves in boilers
Improved High Lift Safety Valve
For low and medium pressure water tube as well as fire tube boilers of different varieties, the safety valve typically used is an Improved High Lift type of safety valve which is a modification of the high lift type safety valve.
Compared to the other safety valves it has some significant modifications like
- Improved waste steam flow because of the removal of valve wings
- Introduction of the floating ring that reduces the risk of seizure.
Each of these spring-loaded safety valves, ordinary, high lift and improved high lift, share a common feature i.e. the shape of the valve seat and a lip on the valve. The valve seat is designed in such a way that the pressure exerted by the waste steam compresses the spring. Moreover, the lip is situated around the valve seat so that, when the valve lid lifts, escaping steam hovers around in the annular space; the resultant build-up of pressure acting upon the greater valve lid area causes the valve to lift abruptly. Moreover, this particular arrangement helps the valve seat to shut off slowly and precisely with minimum blow down effect. Now an interesting feature related to this valve is that the escaping steam enters the waste steam pipe and, although some regulations are there to prevent this pipe being manufactured too small, some build-up pressure prevails. Improved high lift safety valve makes use of this waste pressure so that it can increase the valve lift by the floating ring itself forming an effect of piston; now this pressure acts upon the piston which further compresses the spring and increases the valve lift. Adjustment or setting of these safety valves are done by these steps:
Initially the compression rings are removed, then screw down the compression screws, raise boiler pressure to the required blowoff pressure level, screw back compression screw until valve blows, then screw down the compression screw carefully, tapping the valve spindle downwards very lightly while doing so, till the valve gets closed. When it is finally in place, the split compression rings need to be fitted, after that the hoods, keys, padlocks and finally the easing gear. At last, inspect the easing gear to ensure that it is in good working condition. For a multi-boiler installation, raise all the boiler pressures to the required blow off pressure, make sure the boilers are connected up, then proceed as described above, setting each valve in turn.
Always remember when overhauling safety valve, you must ensure that all the parts are put back in correct order and remember to hang them over a cord and check for any cracks and all by tapping them with a hammer gently. Do not forget to check the drains and easing gear.
Full Bore Safety Valve
These valves are generally mounted on boilers with pressure more than 21 bar(approximate). A lot of issues arise when it comes to spring loaded valves like the feathering effect caused due to the uneven closure of the valve at extreme conditions.
Moreover, distortion of spring is another major concern. Anyways in these cases full bore safety valve comes handy. The valve operation is as follows- The relay valve lifts as soon as the blow down pressure is achieved and hence the excess pressure is relieved to the atmosphere through a series of ports. Steam is then allowed to travel through the connecting pipe, into the cylinder of the main valve, and since the area of piston is about twice that of the valve, the valve opens against boiler pressure. When the boiler pressure falls, the relay valve closes, uncovering as it does so the ports above it and finally the main valve closes. Since the greater the boiler pressure the more rapid will the valve close, and hence the greater the steam saving.
Adjustment of the blow off pressure is carried out by changing the compression on the
control valve spring; this is done by means of an adjusting nut fitted to the control valve.
Easing gear is fitted so that it can open the main valve in case of any emergency situation.
In addition, a hand operated easing lever is also fitted to the control valve which allows the steam to act upon the main piston.
Full Lift Consolidated Type Safety Valve
Generally used for pressure values more than 60 bar, it has various special features like: –
- Adjustable blow-down control ring screwed onto the valve seat provides a
quick full lift or `pop’ action to the opening valve disc, and a cushioning or blow-down
control action as the valve closes.
- Second important feature is a boost cylinder fitted above the valve disc to provide a precision closing control. As the valves are fully open, some escaping steam is bled off to this boost cylinder and it is vented directly to the atmosphere. Hence the resulting down thrust reduces the blow-down effect.
- Another notable feature is the installation of the thermo-disc valve seat. It is like a thin wall providing extra flexibility at the contact point as the valve closes. This feature allows better heat transfer between the mating surfaces and prevents leakage.
The valve operation is as follows: Initially the valve lifts a little bit which leads to some amount of escaping steam acting upon the major area of the valve. Now the pressure created by this escaping steam results the valve to lift till it is open totally. When the valve is fully open the escaping, area becomes equal to the area of supply which is the valve seat in this case. The descriptions of other parts like compression nut, easing gear, valve spring, valve chest is exactly similar to that of an improved high lift safety valve.