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Weir Valves & Controls ExcellentEngineering

Solutions

TECHNICAL BULLETIN

Prepared By: Adrian Crof t Issue Date: 24 /11 / 2005Reference: AR-C2-R0 Pages: 3

Subject:Overboard Dump Valves

Introduction

Overboard dump valves, as the name

suggests, are valves that are typicallyused to control the flow of seawater beingdumped overboard on off-shoreinstallations. Process conditions thatwould be innocuous on standardapplications, become severe when appliedto overboard dump valves.

Often users fail to realise the criticality ofthe application which results valves beingincorrectly specified, ultimately leading tovibration, erosion and failure.

Fig 1. Typical Offshore Installation

Severe Service Application

Overboard dump valves are classed assevere service control valves due to thefact that there is a high potential forcavitation occurring on the valves. Thepotential for cavitation is caused by therelatively high inlet pressures and the low

outlet pressures. Often the process

engineers that specify the valve processconditions state that the valve outletpressure is 1 BAR A, i.e. atmospheric, butin reality as the water is being dumped intothe sea then there will be a vacuum in theoutlet pipe. The discrepancy between thetheoretical pressure and actual pressureoften means that the valve is incorrectlysized, and with such low outlet pressuresa difference of 1 BAR can mean thatsevere cavitation occurs.

What is Cavitation?

Cavitation is a phenomenon that can occurin control valves on liquid service duties. Inits most severe form cavitation can destroya control valve trim in a matter of hours. Itis therefore important to control the level ofcavitation when selecting a control valve.

Cavitation occurs in liquid systems whenlocal pressure fluctuations near the liquidsvapour pressure result in the suddengrowth and collapse of vapour bubbles(cavities) within the liquid. Shown on thegraph below, as the inlet pressure (P1)passes through the valve, and exits thetrim, then if the pressure drops below, andthen rises above the vapour pressure (PV)then these cavities are formed. The cavitycollapse produces a localised shock waveand liquid microjets. If these impact on themetallic surface of the valve then severepitting and erosion damage can occur.

Cavitation often produces high levels ofnoise and vibration across a broad rangeof frequencies. Excessive vibration can

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Technical Bulletin Cont inuation Sheet

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loosen flange bolting, damage pipingsupport structures, and destroy processequipment. The hazards of excessivenoise can create dangerous conditions forpeople and their environment.

Fig 2. Pressure Drop Profile in a Single

Stage Trim

Pipework Solutions

There are many solutions that can be

employed to eliminate the occurrence of

cavitation. The first thing that must be

specified however, is the valve application,

and accurate process conditions.

At the design stage, if the process

engineer realises that the valve is going to

be used on an overboard dumpapplication, then the valve outlet pipework

can be designed with either a vacuum

breaker or an inverted U shaped bend.

These two solutions ensure that a positive

pressure is maintained in the downstream

pipe and therefore the more severe

conditions are avoided.

Vacuum Breakers are essentially two

way valves that can be installed in the

pipework. Once a vacuum forms in thepipe, the breaker opens to allow a positive

pressure in the pipe.

U shaped bend can be installed in the

downstream pipe. The bend ensures a

head of water in the pipe which in turn

maintains a positive pressure at the outlet

of the valve.

Fig 3. Installation of a Downstream Bend

Valve Solutions

If the correct process conditions are

specified at the time of order, then thecontrol valve can be selected to eliminatethe cavitation that might occur.

If there is only one set of processconditions specified, then the simplestmethod of eliminating the potential forcavitation is to install a baffle plate at thevalve outlet. The baffle plate generates abackpressure in the line therebyincreasing the valve outlet pressure. Asbaffle plates have a fixed orifice size then

they are only suitable one set of processconditions. Where the process conditionsvary with the valve opening, then thesolution should be made with the valvetrim as this is a variable orifice.

The most effective valve solution is toinstall a low recovery or multi stage trim inthe valve. Cage guided valves are typicallyvalves with the low pressure recovery. Theflow is broken down into a series of radialjets, and as the jets impinge together in

the centre of the cage then pressure iscontrolled. Depending on the level ofcavitation at the vena contracta then fitting

U shaped bend

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a low recovery trim will mean that thepressure at the vena contracta does notfall below the vapour pressure.

Where cavitation cannot be eliminated bya low recovery trim, then a multi stage trimcan be employed. These are specificallydesigned to eliminate cavitation by thecontrol of the pressure drop through eachsleeve of the trim. The number of sleevesused in the cage depends on the level ofcavitation. High levels of cavitation willrequire more sleeves so that the pressureat each stage of letdown is more graduallyreduced. Each sleeve has a number ofradial holes, with carefully calculated flowareas. The flow area is controlled toapportion the pressure drop across eachstage of the trim. Thus, the small radialjets pass through a tortuous flow pathresulting in frictional and impingment

pressure losses.

Fig 4. Treatment of Cavitation Using theValve Trim

On overboard dump applications, theapportionment of the pressure drop can bemade so that the largest pressure dropsare taken over the initial trim stages, whileat the final stage a minimal pressure dropis taken. This ensures control of the

pressure at the vena contracta andtherefore the elimination of cavitation.

Fig 5. Flow path through a cascade trim