POWER February 2014 - 37

InstrumentatIon & Control
1. Establishing drum level control valve pressure drop. Courtesy: Bechtel
Power Corp.
psi. Ideally, this is the pressure difference,
which should be available between the
pump's head-flow curve and the system
resistance curve (exclusive of the control
valve pressure drop) at the minimum operational
flowrate of 800 gpm.
Note that due to high static head of a
Connell Method. In this method, the
minimum pressure drop assigned to the control
valve is based on pump discharge pressure,
increased frictional pressure drop due
to maximum flow rate, and base pressure
drop to account for the fact that even in the
wide open position the control valve generates
some pressure drop. The Connell correlation
is expressed in the equation below:
2
dP = 0.05Ps +1.1
Q
Q
n
d
1 (e + r) B
+
is the pressure at the beginning of the
system (pump discharge),
Qd
where:
dP is differential pressure,
Ps
is the design flow rate in the line,
Qn is the normal flow rate in the line,
e is the differential pressure (dP) across
the process equipment at normal flow,
r is the dP across only the piping and
valves at normal flow, and
B is the base dP for the control valve.
Minimum Control Valve Pressure Drop
Method for Pumped Application. In this
method, attributed to F.C. Yu, the control valve
in a pumped application is assigned a minimum
pressure drop at maximum design flow rate and
maximum 80% control valve open position (assuming
control valve regulating range is 20%
to 80% valve open position). The control valve
opening is then checked at normal flow to make
sure that the opening is not below the minimum
20% open limit. The minimum pressure drop
is 10 to 15 psi greater than the pressure drop at
valve full-open conditions.
Method Application Examples
Below is the computation used when applying
the various methods for determining the
assigned control valve pressure drop for the
drum level control valve. Note that the values
are not exactly comparative because each
February 2014 | POWER
method uses parameters that are not common
to all three methods.
Traditional Method:
■ Dynamic losses = 75 psi at normal flow
■ Control valve dP = 33% of dynamic losses,
including control valve dP
■ Therefore, dP / (75 + dP) = 0.33, where dP
= 38 psi
Connell Method:
■ Pump discharge pressure = 3,000 psi.
■ Base dP for control valve = 11 psi.
■ Dynamic losses at design flow = 1.2 times
normal flow.
■ Control valve dP = (0.05 x 3,000) + 1.1 x
[(1.2)2 - 1] x 75 +11 = 197 psi.
Minimum Control Valve Pressure Drop
Method for Pumped Application:
■ Obtain valve characteristic curve showing
the coefficient of flow (Cv) versus % valve
opening and check Cv at 80% open (for
example: Cv = 150 at 80% open).
■ Using maximum design flowrate (Q) of
2,300 gallons per minute (gpm) and Cv
value at 80% open, calculate the control
valve dP as follows: Control valve dP = (Q
/ Cv) 2 x specific gravity = (2,300 / 150) 2
x 0.9 = 212 psi. Ideally, this is the pressure
difference, which should be available between
the pump's head-flow curve and the
system resistance curve (exclusive of the
control valve pressure drop) at the maximum
design flowrate of 2,300 gpm.
■ Now, using the same valve characteristic
curve, select the Cv at minimum controllable
20% open (for example: Cv = 45 at
20% open).
■ Assume that the minimum operational
flowrate (for example, 800 gpm) will be
handled by the valve at minimum 20%
open position, calculate the valve dP as
follows: Control valve dP = (Q / Cv) 2 x
specific gravity = (800 / 45) 2 x 0.9 = 284
www.powermag.com
boiler feed pump in a power plant application,
the Traditional Method for establishing
control valve pressure drop provides a
low pressure drop, which is inadequate for
this application. The remaining two methods
(Connell Method and Yu's Method, or Minimum
Control Valve Pressure Drop Method
for Pumped Application) consider this high
static head in the calculation and result in a
more reasonable value. These methods can
be used as the first iteration but should not
be used without additional checks. The additional
checks should be done to ensure
that the selected valve sizing (Cv and dP) is
able to deal with all operating scenarios with
valve opening remaining within the acceptable
controlling range of the valve. For this
purpose, it is essential to recognize the various
operating scenarios and functional considerations
at the power plant, which impact
the control valve dP.
Functional Considerations for
Drum Level Control Valves
The basic functional consideration for a drum
level control valve serving a power boiler is
that it should be capable of covering a wide
range of operating parameters.
Normal Flow Versus Minimum/Maximum
Flow. In a power boiler fitted with a
constant speed motor-driven pump, the drum
level control valve should be capable of handling
the required flow during normal plant
load as well as that required at reduced load
(including startup).
In addition, the drum level control valve
should be capable of handling maximum
flow requirements, which could arise due to
a combination of feedwater flows in addition
to the rated flow, such as boiler blowdown,
sootblowing, or spray water usage.
If startup or low-load operation results in
too severe of a range for one valve, a second
startup control valve may be required to split
the service conditions.
Ability to Restore Drum Level. The
maximum flow requirement established
should be utilized and checked against the
flow requirement when trying to restore
drum level from low level to normal level. In
case the drum level drops to around the lowlevel
mark and the boiler is operating under
full-load conditions, it may not be possible to
restore drum level without reducing load, unless
the feedwater system has been designed
37
http://www.powermag.com
POWER February 2014

Table of Contents for the Digital Edition of POWER February 2014
