The importance of accurate valve sizing

Sizing valves correctly is essential for precision, quality and safety in process industry operations, says Steve Meadows of Asco Joucomatic.

The precision and quality demanded in process operations today mean that the importance of sizing process valves correctly is paramount.

What is not acceptable is the traditional "rule-of-thumb" method where the valve is oversized "just to be on the safe side": in fact, experience shows that oversizing is as bad in its effects as undersizing.

The pitfalls with undersizing are many, including the inability to meet the desired flow requirements; the flashing of liquids to vapours on the outlet side of the valve; a fall in the outlet pressure; and substantial pressure losses in the piping system.

With oversizing, the user is faced with the unnecessary cost of oversized equipment and then has to cope with a number of problems, many of which can impair the operational life of the valves.

These include: variable or erratic control of flow through the valve; oscillating of internal parts to maintain required internal pressure, caused by lack of flow; erratic operation, such as failure to shift position; finally, in some designs, erosion of valve seats, because they operate in the nearly closed position In general, the best approach to avoid the problems of under and oversizing is to know as many of the conditions surrounding the application as possible.

The first parameter is the flow required: in cubic metres per hour (m3/h) for liquids; normal cubic metres per hour for gases and kilograms per hour (kg/h) for steam.

These flow figures can be obtained simply by asking the user his or her requirements; however, to avoid mistakes it is far better to refer to more concrete data such as nameplates on pumping equipment, boiler room charts or calculations.

The second parameter is the inlet pressure (P1); this is obtained from the source of the supply, or, better still, determined by placing a pressure gauge near the valve inlet.

The output pressure (P2) can also be obtained by gauge observations, but is usually tied-in with specifications regarding allowable system pressure drop.

However, if both the inlet pressure and the pressure drop are known, then the outlet pressure is easy to determine.

The size of the pressure drop (dp) across process valves is particularly important in large or complicated systems.

It is desirable to keep the figure to a minimum, and often the customer will have definite specifications concerning the factor.

Of course, if the valve is discharging to atmosphere, the pressure drop is equal to the inlet pressure, when dealing with liquids.

However, when sizing valves for use with gases and steam, although the valve may be discharging to atmosphere only 50% of the inlet pressure can be used for calculation purposes in manufacturer provided formulas (commonly called critical pressure drop).

In all other cases the pressure drop is the difference between inlet and outlet pressures.

Determining the pressure drop also has implications where valves have a stated "minimum operating pressure differential" - a term which is often misunderstood.

Certain pilot operated valves function by differential pressures created internally by "pilot" and "bleed" arrangements.

This differential is measured as the difference between inlet and outlet conditions on all valve constructions.

If pressure conditions are not known, but only flow information, we can use manufacturer-supplied graphs or formulas to solve the resulting pressure drop.

If this is less than the assigned minimum differential, then the valve is oversized.

In these situations, a valve with a lower minimum operating pressure differential should be employed or, alternatively, a smaller valve with a more closely defined Kv factor.

(Kv is the flow coefficient in cubic metres per hour or litres per minute).

Also useful to the process engineer engaged in valve selection is the converse of the minimum operating pressure differential.

When a valve is closed, the supply pressure is present at the inlet port only.

This is the pressure the valve has to open against: the pressure the electrical solenoid has to overcome to open the valve and allow flow to occur.

This pressure is called maximum operating pressure differential or simply MOPD.

The value given for this in manufacturers' catalogues must be equal to or greater than the maximum pressure at the supply port at which the valve must open.

This is not always the same for different types of fluid, and that AC actuated valves usually have higher pressure ratings than DC valves.

Strictly speaking, the MOPD is the maximum pressure drop across the valve when the valve is closed.

If there is a pressure at the outlet port when the valve has to open, this could be subtracted from the inlet pressure to arrive at the MOPD.

However, if at some time a zero pressure is present at the outlet, the valve will have to open at the pressure at the inlet, which may be too high, causing possible coil burnout on an AC source.

Therefore, the supply pressure is considered as the MOPD in conservative designs.

Full information regarding Asco Joucomatic's comprehensive range of solenoid and pressure operated valves for fluid control is contained within a new catalogue produced by the company.

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