78.Boiler mountings

BOILER MOUNTINGS

Various valves and other fittings are required for the proper working of the boiler. Those attached directly to the pressure parts of the boiler are referred to as boiler mountings. In general all these mountings must be carefully designed to perform their function correctly. They must be positioned so as to be readily accessible both for maintenance and operation, the later being performed either directly, or indirectly by means of extended rods, spindles, etc. For a water tube boiler the mountings usually consists of the following items:

 

Safety Valves

These are fitted to protect the boiler from the effects of overpressure. The DOT demand that at least two safety valves are fitted to each boiler, but in practice it is usual to fit three safety valves – two on the steam drum, and one on the steam drum, and one on the super heater outlet header. This latter valve must be set to lift before the drum safety valves so as to ensure a flow of steam through the super heater under blow off conditions, it is normally of the same basic type fitted on the drum.

Main Stop Valve

This is mounted on the super heater outlet header and enables the boiler to be isolated from the steam line. If two or more boilers are fitted supplying steam to a common line, the stop valve on each boiler must be a screw down, non-return type. This is to prevent steam from the other boilers flowing into a damaged boiler in the event of a loss of pressure due to a burst tube. In some cases the main stop valve incorporates an automatic closing device, designed to operate in emergency conditions, to shut off steam from the main turbines.

Auxiliary Stop Valves

This is basically a smaller version of the main stop, fitted for the purpose of isolating the boiler from the auxiliary steam lines. Again these must be screw down, non- return type valves if necessary to prevent steam flowing back into the boiler in the event of damage. The valve will be mounted on the super heater outlet header; a de-super heater can be used to reduce the steam temperature as required.

Feed Check Valves

These are fitted to give final control over the entry of feed water into the boiler. They must be screw down, non-return valves so that in the event of a loss of feed pressure the boiler water cannot blow back into the feed line. Main and auxiliary feed checks are fitted. The main check is often fitted to the economizer inlet header; if not, like the auxiliary check, it will be mounted directly on the steam drum. Extended spindles are usually fitted so the valves can be operated from a convenient position. Care must be taken to ensure the valve can be operated easily and quickly, and that a positive indication of the open and closed positions for the valve is given.

Boiler Feed Water Regulator

The water level in a boiler is critical. If it is too low, damage may result from overheating; too high and priming cab occur with resultant carry over of water and dissolved solids into super heater, steam lines, etc. Automatic feed regulators are therefore fitted to control the flow of water into the bolero and maintain the water level at its desired value.

They are fitted in the feed line, before the main feed check. In most cases they use a float or thermal means of operation and thus must have a direct connection to the steam and water spaces as required. The regulator can be attached directly to the boiler shell, or alternatively mounted in an external chamber with balance connections to the steam drum, or boiler shell. In the case of water tube boilers with their high evaporation rate and small reserve of water the control of the water level is so critical that the classification societies demand that some form of automatic feed regulator must be fitted.

Water Level Indicators

The DOT demand that at least two water level indicators must be fitted to each boiler. In practice the usual arrangement consists of two direct reading water level gauges mounted on the steam drum, and a remote reading indicator placed at a convenient control position.

Low Water Alarms

The classification societies demand that these should be fitted to reduce the risk of damage in the event of a loss of water in the boiler due to a burst tube or failure of the feed supply. In some cases they are mounted inside the steam drum, but many are mounted externally. Various types are available, either steam or electrically operated. Some versions also incorporate high water level alarms.

Blow Down Valves

These are fitted to the water drum to enable water to be blown from the boiler in order to reduce the density. When the boiler is shut down these valves can be used to drain it. They usually consist of the valves mounted in series, arranged so that the first valve must be full open before the second can be cracked open; i.e., sufficient to give the required rate of blow down. In this way the seating of the first valve is protected from damage, so reducing the risk of leakage when the blow down valves are closed. These blow down valves discharge into a blow down line leading to a shipside discharge valve.

Scum Valves

These should be fitted when there is a possibility of oil contamination of the boiler. They are mounted on the steam drum, having an internal fitting in the form of a shallow pan situated just below the normal water level, with which to remove oil or scum from the surface of the water in the drum. These valves discharge into the blow down line.

Drain Valves

These are fitted to headers, etc., so enabling the boiler to be completely drained. They must not be used to blow down, only being opened when the boiler is shut down.

Air Vents

These are fitted to the upper parts of the boiler as required to release air from drums and headers, either when filling the boiler, or raising steam.

Super Heater Circulating Valves

These are fitted so that when raising steam they can first release air from the super heater, and then provide enough circulation to prevent overheating by allowing sufficient steam to blow off to the atmosphere or to a suitable drain system. They should only be closed when there is enough demand for superheated steam to provide the minimum circulation of steam required to prevent overheating.

Chemical Dosing Valves

These are fitted to the steam drum to enable feed treatment chemicals to be injected directly into the boiler.

Salinometer Valves

These are fitted to the water drum to enable samples of boiler water to be drawn off so that the tests required for the control of the feed treatment can be carried out. At high pressures it is necessary to provide some means of preventing flash off taking place as the pressure over the sample is reduced to atmospheric. This is usually done by passing the water from the salinometer valve through a cooling coil, which reduces its temperature to a value below 100 degrees Centigrade.

Soot Blower Master Steam Valves

These are usually mounted on the super heater outlet header to ensure the super heater is not starved of steam while blowing tubes. In some cases two valves are fitted in series, with a drain valve between them in order to prevent steam leaking into the soot blower steam supply lines when these are not in use.

Pressure Gauge Connections

Fitted as required to steam drum, super heater outlet header, etc., to provide the necessary pressure readings. In addition suitable connections must be provided for the pressure sensing points required for automatic combustion control systems, etc.

Thermometers

Pockets must be provided in super heater headers, etc. for the fitting of either direct or remote reading thermometers.