net Archive. Yahoo! is not affiliated with the authors of this page or responsible for its content.
Handbook OZ1000 7/00 Bulletin OZ1000
7/00
Masoneilan Control Valve
Sizing Handbook
DRESSER
VALVE
DIVISION
Masoneilan 2
DRESSER
VALVE
DIVISION
Masoneilan
Table of Contents
Flow Coefficient ...................................................................
3
Operating Conditions ..........................................................
3
Specific Gravity ....................................................................
3
Pressure Drop Across the Valve ........................................
4
Flowing Quantity ..................................................................
4
Liquid Flow Equations .........................................................
5
Liquid Pressure Recovery Factor .......................................
6
Combined Liquid Pressure Recovery Factor ....................
6
Cavitation in Control Valves ...............................................
6, 7
How to Avoid Cavitation ......................................................
7
Effect of Pipe Reducers .......................................................
7
Equations for Nonturbulent Flow .......................................
8
Gas and Vapor Flow Equations ..........................................
9
Multistage Valve Gas and Vapor Flow Equations .............
10
Ratio of Specific Heats Factor ............................................
10
Expansion Factor .................................................................
10
Two Phase Flow Equations .................................................
11
Choked Flow .........................................................................
12
Supercritical Fluids ..............................................................
12
Compressibility ....................................................................
13-14
Thermodynamic Critical Constants ....................................
15-16
Liquid Velocity in Steel Pipe ...............................................
17
Steam or Gas Flow in Steel Pipe ........................................
18-19
Commercial Wrought Steel Pipe Data ................................
20-21
Properties of Steam .............................................................
22-27
Temperature Conversion Table ..........................................
28
Masoneilan Control Valve Sizing Formulas .......................
29-30
Metric Conversion Tables ...................................................
31-32
Useful List of Equivalents ...................................................
33
References ............................................................................
33
Note: Tables for C
v
, F
L
, x
T
and K
c
vs Travel are found in publication
Supplement to Masoneilan Control Valve Sizing Handbook OZ1000.
Engineering Data
Particulars contained in this publication are for general information only and Masoneilan reserves the right to modifiy the contents without prior
notice. No warranty either expressed or implied is either given or intended.
© 2000 Dresser Industries, Inc. All rights reserved. 3
DRESSER
VALVE
DIVISION
Masoneilan
Foreword
Specific Gravity
In the flow formulas, the specific gravity is a square root
function ; therefore, small differences in gravity have a
minor effect on valve capacity. If the specific gravity is not
There is no substitute for good engineering
judgement.
Most errors in sizing are due to incorrect
assumptions as to actual flowing conditions. Generally
speaking, the tendency is to make the valve too large to
be on the "safe" side (commonly referred to as
"oversizing"). A combination of several of these "safety
factors" can result in a valve so greatly oversized it tends
to be troublesome.
The selection of a correct valve size, as determined by
formula, is always premised on the assumption of full
knowledge of the actual flowing conditions. Frequently,
one or more of these conditions is arbitrarily assumed. It
is the evaluation of these arbitrary data that really
determines the final valve size. No formulas, only good
common sense combined with experience, can solve
this problem
.
Operating Conditions
This handbook on control valve sizing is based on the use
of nomenclature and sizing equations from ISA Standard
S75.01 and IEC Standard 534-2. Additional explanations
and supportive information are provided beyond the
content of the standards.
The sizing equations are based on equations for predicting
the flow of compressible and incompressible fluids through
control valves. The equations are not intended for use
when dense slurries, dry solids or non-Newtonian liquids
are encountered.
Original equations and methods developed by Masoneilan
are included for two-phase flow, multistage flow, and
supercritical fluids.
Values of numerical factors are included for commonly
encountered systems of units. These are United States
customary units and metric units for both kilopascal and
bar usage.
The principal use of the equations is to aid in the selection
of an appropriate valve size for a specific application. In
this procedure, the numbers in the equations consist of
values for the fluid and flow conditions and known values
for the selected valve at rated opening. With these
factors in the equation, the unknown (or product of the
unknowns, e.g., F
p
C
v
) can be computed. Although these
computed numbers are often suitable for selecting a
valve from a series of discrete sizes, they do not represent
a true operating condition. Some of the factors are for the
valve at rated travel, while others relating to the operating
conditions are for the partially open valve.
Once a valve size has been selected, the remaining
unknowns, such as F
p
, can be computed and a judgement
can be made as to whether the valve size is adequate. It
is not usually necessary to carry the calculations further
to predict the exact opening. To do this, all the pertinent
sizing factors must be known at fractional valve openings.
A computer sizing program having this information in a
database can perform this task.
The use of the flow coefficient, C
v
, first introduced by
Masoneilan in 1944, quickly became accepted as the
universal yardstick of valve capacity. So useful has C
v
become, that practically all discussions of valve design
and characteristics or flow behavior now employ this
coefficient.
By definition, the valve flow coefficient, C
v
, is the number
of U. S. gallons per minute of water that will pass
Flow Coefficient C
v
know accurately, a reasonable assumption will suffice.
The use of .9 specific gravity, for example, instead of .8
would cause an error of less than 5 % in valve capacity.
through a given flow restriction with a pressure drop of
one psi. For example, a control valve that has a maximum
flow coefficient, C
v
, of 12 has an effective port area in the
full open position such that it passes 12 gpm of water with
one psi pressure drop. Basically, it is a capacity index
upon which the engineer can rapidly and accurately
estimate the required size of a restriction in any fluid
system. 4
DRESSER
VALVE
DIVISION
Masoneilan
Pressure Drop Across the Valve
On a simple back pressure or pressure reducing
application, the drop across the valve may be calculated
quite accurately. This may also be true on a liquid level
control installation, where the liquid is passing from one
vessel at a constant pressure to another vessel at a lower
constant pressure. If the pressure difference is relatively
small, some allowance may be necessary for line friction.
On the other hand, in a large percentage of control
applications, the pressure drop across the valve will be
chosen arbitrarily.
Any attempt to state a specific numerical rule for such a
choice becomes too complex to be practical. The design
drop across the valve is sometimes expressed as a
percentage of the friction drop in the system, exclusive of
the valve. A good working rule is that 50% of this friction
drop should be available as drop across the valve. In
other words, one-third of the total system drop, including
all heat exchangers, mixing nozzles, piping etc.., is
assumed to be absorbed by the control valve. This may
sound excessive, but if the control valve were completely
eliminated from such a system, the flow increase would
only be about 23%. In pump discharge systems, the head
characteristic of the pump becomes a major factor. For
valves installed in extremely long or high-pressure drop
lines, the percentage of drop across the valve may be
somewhat lower, but at least 15% (up to 25% where
possible) of the system drop should be taken.
Remember one important fact, the pressure differential
absorbed by the control valve in actual operation will be
the difference between the total available head and that
required to maintain the desired flow through the valve. It
is determined by the system characteristics rather than
by the theoretical assumptions of the engineer. In the
interest of economy, the engineer tries to keep the control
valve pressure drop as low as possible. However, a valve
can only regulate flow by absorbing and giving up pressure
drop to the system. As the proportion of the system drop
ac