Lot of accidents are reported while repair works or during dismantling of equipment and pipe lines containing flammable products in liquid or gaseous forms. Similarly a phenomenon called "water hammer" occurs while admitting steam to the pipe lines which contain condensed water inside.
What is Water Hammering
Consider a steam pipe line containing condensed water in it. When admitting steam directly to this pipe line, water inside cools down part of the steam which creates a vacuum inside the line since the volume occupied by the steam is larger than that of water. Water remains in the line then rushes to fill the vacuum with very high velocity. This water with very large kinetic energy can create huge hydraulic force that may heavily damage causing leakages or even cracks while hitting bends, T-joints, valves, etc.
How to Prevent Water Hammering
Before starting steam to admit, open drain valve and drain the line completely. Practically it is not possible to drain water completely by gravity. Hence a small sized preheating valve is fitted in parallel with main steam stop valve. During starting this preheating valve is opened first which admits steam in very small quantity to the lines, thereby pushing the water outside slowly through the drain. Once, only steam starts coming through the drain, start opening main valve very slowly. Now drain valve can be closed. A smooth operation as described above can eliminate disastrous water hammering.
Even though we can reduce the effect of water hammering, it cannot be eliminated. So the steam line system must be designed for withstanding hydraulic forces generated by the same. In the event of hammering weak links in the system are more prone to damage. This include gaskets between pipe line flange connections, gland packing for stuffing box of the valves, gasket between valve bonnet and body, pressure gauge connections, blanks in the system, etc. Normal gaskets are not used for steam system. All boiler mountings must use metal gaskets. Metal gaskets are best in sealing and strength point of view. Similarly proper gland packing in adequate amount to be packed in the valve stuffing boxes.