1.7 Water hammer in Deaerator

 

Water Hammer:

Introduction

Water hammer is a pressure surge or wave caused when a fluid, usually a liquid but sometimes also a gas, in motion is forced to stop or change direction suddenly (momentum change).

When condensate is formed in steam line, the flow inside the pipe has two components-

i)                   Steam and

ii)                 Condensate.

            The flow velocity of steam is much higher than that of the condensate. During such dual phase flow, the heavy condensate which flows at the bottom of the pipe is pulled by high speed steam. This forms the water slug which is much denser than steam and travelling with the same velocity of steam. When this slug is stopped by any abruption like a bend or equipment, the kinetic energy of the slug will be suddenly converted into pressure energy which will create a shock wave in the entire pipe work. The pipe work will keep on vibrating until this energy is dissipated in the structure. 

Best practices to avoid water hammer:

Though we cannot be eliminated the water hammer completely from steam systems, but it can certainly be avoided. There are some best practices, which when followed, ensure least chances of occurrence of water hammer.

1.      There should always be a gradual slope (gradient) in direction of flow. Steam lines

2.      Steam traps should be installed at regular intervals and also at the lower points.

3.      Sagging of pipes should be avoided by providing proper support in pipe lines.

4.      Drain pockets should be properly sized to ensure that all the condensate reaches the trap.

5.      Isolation valve should be open slowly during the start-up.

6.      Eccentric reducers should be used against concentric reducers in horizontal position.

7.   Recommended velocity of saturated steam and water should be 20-35 m/s and 2-3m/s respectively in pipe network.

Water hammer in Deaerator:

Water hammer in Deaerator can be caused due to some of the following reasons:

1.      During initial steam charging in deaerator.

2.      When turbine trip or other event causing a pressure dip in deaerator. The deaerator pressure falls, where upon steam forms in down comer. As the stem moves up, it leaves a void behind. There after mass of cooler water from the DA storage tank rushes to the pocket. A descent of water for 12-25 Meter causes heavy impact where the descending water column meets the stationary water column in down comer. This phenomenon is shown in below figure.

 

 

References - forbesmarshall.com/ Water-Hammer