Pluralidad de contratos diferentes

The user is able to enter the flow rate and head loss information about a component which is to be included in the pipeline system. Pipe Flow Expert generates a performance curve for the component to allow the effect of the component to be modeled.

Cv and Kv Flow Coefficients

Many manufactures of control valves choose to publish a Cv flow coefficient or a Kv flow coefficient to describe the flow / pressure loss characteristics of their control valves in a standardized manner.

Cv Flow Coefficients:

A Cv flow coefficient specifies the amount of water at 60°F (15.55 °C) in US gpm that will flow

through a valve and produce a 1.0 psi pressure drop. Thus a Cv flow coefficient of 10 indicates that a 1.0 psi pressure drop will occur with a 10 US gpm of water throughput through the valve.

The Cv flow coefficient of a control valve can be calculated from the flow rate and the pressure drop through the valve. For liquids other than water the ratio of the fluid density to the density of water must also be used in the calculation.

Where:

Cv = flow coefficient Q = flow rate in US gpm

∆P = pressure loss in psi across the valve

SG = the ratio of the fluid density to the density of water

With a known Cv flow coefficient the above formula can be re-arranged to calculate the pressure loss for a particular flow rate thus:

Where:

Cv = flow coefficient Q = flow rate in US gpm

∆P = pressure loss in psi across the valve

SG = the ratio of the fluid density to the density of water

The pressure loss through a fitting or valve may also be calculated from:

Where:

h fluid = head of fluid in ft.

K = flow coefficient of a valve or fitting V = fluid velocity entering the fitting in ft/s g = acceleration due to gravity in ft/sec2 also

Where:

P = pressure in psi h fluid = head of fluid in ft.

D = density of fluid in lbs/ft3

Thus a 1.0 psi pressure drop is equivalent to 2.31 ft head of water at Normal Temperature & Pressure (NTP) or 62.3 lbs/ft3.

When a pipe diameter is known it is possible to establish a flow velocity from the Cv flow coefficient in US gpm for a 1 psi pressure drop. Thus it is possible to calculate an equivalent fitting ‘K’ factor which will produce the same pressure loss as the control valve Cv rating.

Pipe Flow Expert uses the equivalent fitting ‘K’ factor method to model the flow and pressure loss through a control valve where a Cv flow coefficient is used to specify the control valve characteristics. A change to the pipe diameter would result in a change to the value of the equivalent fitting ‘K’ factor. Pipe Flow Expert re-calculates the equivalent fitting ‘K’ factor for the current pipe diameter and the fluid density at the start of the solution calculation.

The calculation helper provided on the Cv component dialog uses the flow rate and pressure loss entered by the user, together with the current fluid density to calculate a Cv value to match the specified requirements.

Cv = 0.865 Kv (or more accurately Cv = 0.86497767 Kv)

Kv Flow Coefficients:

A Kv flow coefficient specifies the amount of water at 20°C (68 °F) in m3/hour that will flow through a valve and produce a 1.0 bar pressure drop. Thus a Kv flow coefficient of 10 indicates that a 1.0 bar pressure drop will occur with a 10 m3/hour of water throughput through the valve.

The Kv flow coefficient of a control valve can be calculated from the flow rate and the pressure drop through the valve. The density of the liquid in kg/m3 must also be used in the calculation.

Where:

Kv = flow coefficient Q = flow rate in m3/hr

∆P = pressure loss in bar across the valve D = the density of the fluid in kg/m3

1000 = the density of water in kg/m3

The usual arrangement of the formula for calculation of Kv is shown above.

It can be seen that this formula is similar to the one which is used for calculation of Cv values. With a known Kv flow coefficient, the above formula can be re-arranged to calculate the pressure loss for a particular flow rate thus:

Where:

Kv = flow coefficient Q = flow rate in m3/hr

∆P = pressure loss in bar across the valve D = the density of the fluid in kg/m3

1000 = the density of water in kg/m3

Where:

h fluid = head of fluid in meters.

K = flow coefficient of a valve or fitting V = fluid velocity entering the fitting in m/s g = acceleration due to gravity in meters/sec2 also

Where:

P = pressure in bar

h fluid = head of fluid in meters

D = density of fluid in kg/m3

g = acceleration due to gravity in meters/sec2

When a pipe diameter is known it is possible to establish a flow velocity from the Kv flow coefficient in m3/hr for a 1 bar pressure drop. Thus it is possible to calculate an equivalent fitting ‘K’ factor which will produce the same pressure loss as the control valve Kv rating.

Pipe Flow Expert uses the equivalent fitting ‘K’ factor method to model the flow and pressure loss through a control valve where a Kv flow coefficient is used to specify the control valve characteristics. A change to the pipe diameter would result in a change the matching equivalent fitting ‘K’ factor. Pipe Flow Expert re-calculates the equivalent fitting ‘K’ factor for the current pipe diameter and the fluid density at the start of the solution calculation.

The calculation helper provided on the Kv component dialog uses the flow rate and pressure loss entered by the user, together with the current fluid density to calculate a Kv value to match the specified requirements.

1.000 Kv = 1.156 Cv

The user should be aware that the Cv or Kv flow coefficient specifies the flow rate of water for a particular pressure loss.

When the fluid density is greater or less than water, a different flow rate will be required to produce a 1.00 psi or a 1 bar pressure loss through the valve.

CAUTIONS: Choked Flow:

If the fluid is a gas and the pressure drop exceeds 50% of the inlet pressure to the valve, the flow

will become choked and it will not be possible achieve the calculated flow rate.

Pipe Flow Expert does not model changes in the gas characteristics due to pressure or temperature changes.

To correctly model the pressure drop for the entered Cv or Kv value the density of the gas used in the calculation must be the density of the gas at the outlet of the valve/component.

In effect this means that the fluid data for the fluid zone associated with the control valve must be defined for the approximate pressure condition at the outlet of the valve/component.

The pressure drop for the gas flow rate through the valve, based on the entered Cv or Kv flow coefficient and the fluid zone density in the pipe will be calculated.

The calculated pressure drop accounts for the effect of the expansion of the gas through the valve since using the approximate fluid density of the gas at the valve outlet (as specified in the fluid zone associated with the pipe) during calculation of the solution, caters for this characteristic.

If the fluid zone associated with the control valve does not represent the pressure condition at the outlet of the valve/component, it may be necessary to use an adjusted Cv (or Kv) value for valve selection to take in to account the effect of the gas expansion.

The adjusted Cv (or Kv) value should be based on the Cv or Kv formula for sub critical gas pressure drop.

A simplified version of the Cv formula for sub critical gas pressure drop is shown below:

Where:

Cv = flow coefficient

SCFH = flow rate in ft3/hr (NTP) Dn = the gas density in lbs/ft

3

at 0.00 psig

o

F = gas temperature

∆p = pressure loss in psi absolute po = valve outlet pressure in psi absolute

A simplified version of the Kv formula for sub critical gas pressure drop is shown below:

Where:

Kv = flow coefficient

Q = flow rate in m3/hr (NTP) Dn = the gas density in kg/m

3

at 0.00 barg

o

C = gas temperature

∆p = pressure loss in bar absolute po = valve outlet pressure in bar absolute

Please refer to an appropriate text book for a more detailed formula to take account of piping geometry or gas compressibility, should this be necessary.

The Cv (or Kv) flow coefficient of a control valve is usually stated for the fully open flow condition. The Cv (or Kv) flow coefficient will be less when the valve is partly closed.

In an actual system it is important to select a control valve which has an appropriate Cv (or Kv) flow coefficient for the actual valve position that will be used. A control valve that is too small or too large will never be able to provide the correct control in a system.

Most control valve manufactures recommend that you should select a valve where the required Cv (or Kv) value falls between 20% - 80% of the port opening.

Some control valve manufactures recommend that an allowance of 30% should be added to the calculated Cv (or Kv) flow coefficient to obtain the minimum full flow Cv (or Kv) flow coefficient rating which the selected valve should have (when fully open).

Please check your control valve selection with the control valve manufacturer.