hammering issues.pdf

8/14/2019 hammering issues.pdf

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Water Hammer and pulsations

Because liquid is essentially incompressible any energyapplied to it is transmitted instantly.

If a moving column of liquid is slowed down suddenly by,for example, a quick-closing valve, the sudden change in

liquid velocity in the delivery line creates a pressurewave.

The pressure wave, travelling somewhere between 1000an 1300 m/s, travels backwards up the line to the end of

the pipe where it will reverse direction and travel backtowards the valve.

Depending upon valve size and system conditions, avalve closing in 1.5 s or less can produce a pressure

spike five times the system working pressure leadingto blown diaphragms, seals and gaskets and alsocatastrophic system component failure in transmitters,meters and gauges.


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Water Hammer Stopped Flow

If a moving column of liquid is

slowed down suddenly by, for

example, a quick-closing valve, the

sudden change in liquid velocity inthe delivery line creates a pressure

wave.

The pressure wave, travellingsomewhere between 1000 an 1300

m/s, travels backwards up the line

enlarging the pipe.

Large diameter riser

Branch

Quick closure

Valve closed

Shock

Large diameter riser

Branch

Normal flow

Open valve

Flow

Pressure wave enlarges pipe

Because liquid is essentially

incompressible any energy applied

to it is transmitted instantly.


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Water Hammer Stopped Flow

At the end of the pipe it reverses

direction and travels back towards

the valve.

A valve closing in 1.5 s or less can

produce a pressure spike five times

the system working pressure leading to blown diaphragms, seals

and gaskets and also catastrophic

system component failure in

transmitters, meters and gauges.

Reflected pressure wave

Pressure wave reaches valve


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Water Hammer from condensate

Condensing steam Heat loss

Steam

Sub-cooled condensate Bernoulli effectdraws up wave

Accumulation of condensate is trapped in a portion of

horizontal steam piping.

The velocity of the steam flowing over the condensate

causes ripples in the water.Turbulence builds up until the Bernoulli effect draws up a

wave.


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A general rule-of-thumb is:

where:

P = increase in pressure (bar)

v = flow velocity (m/s)

t = valve closing time (s)

L = upstream pipe length (m)

PI= inlet pressure (bar)

IP

t

Lv052.0P


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Example: assume a solenoid valve having a closure time

of approximately 40 ms, connected to a 15 m long

upstream pipe.

The water flow is 3 m/s and the inlet delivery pressure is4 bar.

What is the amplitude of the pressure spike?

IPt

Lv052.0P

41040

153052.0

P 3

= 62.5 bar


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The wave seals the pipe producing an isolated

pocket of steam.


Isolated steam pocket Heat loss

Steam

Sub-cooled condensate Wave seals pipe


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The collapsing steam void produces a slug of

condensate that is carried along by the steam flow.


Collapsing steam void

5 bar Steam


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The resulting implosion produces a slug of condensate

that can travel at the speed of the steam (up to 160

km/hr)

This will strike the first elbow in its path with a forcecomparable to a hammer blow.


Rebounding wave

Implosion


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NPS 24 line displaced,

supports damaged

NPS 8 branch ripped

from header


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Water Hammer solutions

In the previous example where the solenoid valve

closure time was 40 ms, increasing the time would have

a dramatic effect.

What is the amplitude of the pressure spike if the closuretime is increased to 1 s?

IPt

Lv052.0P

45.1

153052.0

P

= 5.6 bar


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Where water hammer and pulsations cannot be reduced,

use could be made of a snubber (either a sintered filter

or small-bore restriction) inside the pressure connection.

Although very effective in absorbing high frequencyshock pressures, there is a trade off with the dynamic

measurement response.

Furthermore, over time the snubber could become

blocked from a build up of contaminant particles.


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Use can also be made of a pulsation dampener or surge

suppressor.

Typically a pulsation dampener is a hydro-pneumatic

dampener comprising a pressure vessel containing acompressed gas, generally air or nitrogen, separated

from the process liquid by a bladder or diaphragm.

The dampener is installed as close as possible to the

pump or quick closing valve and is charged to 85% of

the liquid line pressure.


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Air/gas

Liquid

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hammering issues.pdf