S. P. B. Lemmers, Y. F. HAU and P.
C. TAN, Dialog E&C Sdn Bhd, Kuala Lumpur, Malaysia
Within hydrocarbon storage terminals,
product pumps are the most vital rotating equipment for terminal operations and
maintenance. When these pumps are not operating adequately, terminal inter-tank
transfer and tank recirculation will occur less efficiently, and shipping
operations may face increased loading times and subsequent demurrage costs.
These pumps are a
significant portion of the total installed cost of the facility. Therefore,
they require adequate maintenance and operation attention in addition to (per
design) protection via penultimate process control and ultimate protection via
safety integrity functions.
When (for operational
flexibility and improved power efficiency) the design includes a variable-speed
product pump, the pump’s protective process control and safety integrity
functions should complement this flexibility in pump operation to take full
advantage of this investment.
This article describes the application
of variable-speed product pumps in hydrocarbon storage terminals, and the optimal
usage of the flexibility introduced by the performance map of these pumps by enhancing
the pump process control and protection systems. The following are
This article draws from
recent experiences in the engineering, design and commissioning of a
hydrocarbon storage terminal with variable-speed pumps. The variable-speed centrifugal
product pumps, their controls and protective systems in this commissioned
terminal have been operating without issue for more than a year.
PUMP CONTROL SCHEMES
Two main process control
schemes are involved in centrifugal pump control: capacity control and
protective control, including emergency shutdown.
Typical examples of pump
capacity control include: no control and a pump float on system resistance, control
by discharge control valve and recirculation control. These typical capacity
control mechanisms are applied to both fixed-speed and variable-speed machines.
Equivalently, there are
typical types of pump protective control schemes and protection via mechanical
devices. These include (among others) mechanical minimum flow valves, orifices,
or automated protection via flow switches or minimum flow control loop.
These pump protection
schemes are designed to ensure that the pump does not run below its minimum
continuous stable flow (MCSF). Running the pump below this flow can result in unacceptable
vibration and subsequent loss of pump mechanical integrity.
Capacity control. The example product pump in
this article is driven by a variable-speed drive motor. The capacity control,
volumetric flowrate and delivered differential head are, therefore, a function
of the pump speed. Additionally, the facility configuration includes a
discharge valve, which can be used to throttle the pump at any speed to move
the operating point to the left or right of the pump curve at the respective
A comparison is made between
a variable-speed driven pump and a fixed-speed driven pump, where the discharge
valve similarly can throttle the pump up and down its single-speed pump curve.
Protective control. Centrifugal pumps require
a certain minimum flow for stable operation. This minimum continuous stable flow
is the lowest flow at which the pump can operate without exceeding acceptable
vibration standards. On the right side of this minimum continuous stable flow on
the pump curves shown in FIGS.
1 and 2
is the preferred operating region around the best efficiency point and rated
flow. The minimum stable continuous flow can be as low as one third of the
rated flow and is always determined by the original equipment manufacturer
The product pump described within
the hydrocarbon terminal has been provided with an automated minimum flow control
loop. This control loop consists of the flow measurement directly at the pump
discharge (the initiating element), a logic solver in the process monitoring
and control system (PMCS), and a minimum flow control valve (the final element).
The minimum flow control
loop controls the flow from the pump discharge to the suction to maintain flow
through the pump above the minimum continuous stable flow—this happens by manipulating
the minimum flow control valve.
The minimum flow control
loop is required for starting and stopping the pump without potential damage.
During startup, the minimum flow control valve is fully open, ensuring the flow
through the pump is above the minimum flow. With a closed discharge valve, the
minimum flow control maintains the pump flow at a stable minimum flow. When the
pump is stopped and ramped down, the minimum flow controller again maintains
minimum flow and will fully open when the pump is stopped.
In case of failure of the
protective control loop, an emergency low-low flow trip should be implemented
to ensure protection of the pump. The architecture of this safety integrity function
(SIF) is an outcome of a safety integrity level (SIL) classification. Through
the emergency shutdown (ESD) system, this trip signal will immediately open the
minimum flow control valve for pump protection and/or trip the pump.
Variable-speed vs. fixed-speed pump control. When pumps are
equipped with a variable-speed drive, there is no single minimum flow point, but
rather multiple minimum flow points, as a function of the pump speed. These
multiple minimum flow points constitute a minimum flow line.
The main reason for the application
of a variable-speed product pump is that hydrocarbon storage terminals can
handle a wide range of products—depending on the pumping rate and resistance in the system, the
pump must deliver and accommodate a demanding and wide range of discharge conditions.
A minimum flow controller adjusts
the minimum flow recycle control valve in the bypass to ensure the flow does
not fall below the minimum continuous stable flow. During normal operation, the
minimum flow control valve is closed and the pump is operating above the
minimum flow. Cavitation in minimum flow control valves can occur if the vapor
pressure of the fluid in the suction vessel is the same order of magnitude as
the operating pressure. In this case, to avoid damage to the minimum flow
control valve, anti-cavitation stages can form part of the minimum flow control
valve design. In addition to cavitation, noise reduction measures can be
installed when required. The OEM of the minimum flow control valve will specify
The difference between
minimum flow control of a fixed-speed pump with one minimum flow point (FIG. 1) and the
minimum flow control of a variable-speed pump with a minimum flow control line (FIG. 2) as a function
of pump speed are depicted.
In FIG. 1, a forward
flow controller is used to maintain the fixed-speed pump on the preferred
operating point of the performance curve. Mechanical energy is wasted over the
discharge flow control valve and introduces additional operational cost.
FIG. 2 depicts the functionality of variable-speed
control. As a function of the required flowrate, the operator can increase the
pump speed and the pump will follow the system resistance curve to find an
operating point where its performance map meets the resistance curve. Therefore,
no mechanical pump energy is wasted over a discharge flow control valve or motor-operated
MINIMUM FLOW CONTROL VALVE
SPECIFICATION AND DESIGN
The operational benefits of high-turndown
ratio. It is a common belief and potential misconception
that asking for more immediately means more cost. The turndown ratio is the
ratio between the normal maximum system flow and the minimum controllable flow,
and it refers to the installed valve’s flow characteristics.
control valve that provides pressure control at low opening as well as capacity
for high flowrates is advantageous to the process control dynamics of the
Based on two operational
scenarios, this section describes why specifying more turndown immediately is
operationally lucrative and beneficial. As an example, assume two minimum flow
control valves: one with low turndown, and one with high turndown.
These control valves have
an identical overall valve flow coefficient (Cv), which is defined as the flow
capability of a control valve at fully open conditions relative to the pressure
drop across the valve. Cv flow coefficients are the number of liters per
minute (l/min) of water that will pass through a given orifice or passage at a
pressure drop of 1 bar; of, in imperial units, the flow coefficient Cv is the volume (in U.S. gallons) of water at 60°F (16°C)
that will flow per minute through a valve with a pressure drop of 1 psi
(6.9 kPa) across the valve.
Simplified formulas according to Fluid Controls
Institute Inc. Standard FCI 62-1 are presented in TABLE 1.
The characteristics of the
two example flow control valves (FCV) valves—one with low turndown (TABLE 2) and one with high turndown (TABLE 3)—are depicted here as a function
of their opening or travel (0% is closed; 100% is open).
These examples apply to
both fixed-speed and variable-speed pumps. For both driver types (fixed and
variable), it is recommended to ask for high turndown, which is explained through
two operational scenarios below.
high turndown for both minimum and maximum speeds for the variable-speed pump
is important to ensure the operational flexibility associated with high
turndown is available throughout the complete variable-speed pump performance
scenario 1. This operational
scenario assumes a sudden, inadvertent or intended closure of the pump
discharge valve, or the initiation of a pump protection emergency trip. In both
cases, the minimum flow control valve must be opened either via the control
system (PMCS) in case of valve closure or via a trip in case of an emergency
control valve with low turndown (TABLE 2) already takes up 40% of the rated Cv value of the
minimum flow valve at 10% travel of the opening, and the associated flowrate
equals 200 m3/hr. The minimum flow control valve with the high
turndown (TABLE 3)
only uses 10% of its same overall Cv value and 10% travel, and the associated
flowrate is only 60 m3/hr (e.g., ~10% of the normal flow of 630 m3/hr).
this happens, the opening of the high-turndown minimum flow control valve results
in a smoother and more controlled pump stop and lower overall disturbance to
the process dynamics.
Operational scenario 2. When a trading company
becomes a client at a hydrocarbon storage terminal, there will be an operational requirement to completely
empty the tank to ensure that all inventory belonging to that client is
tradable and pumpable. This is called stripping the tank—to successfully enable this stripping operation,
hydrocarbon storage tanks are provided with a sloped bottom floor and sump.
hydrocarbon products with a high vapor pressure, the stripping operation can be
quite cumbersome and delicate. In the event a vapor pocket is drawn due to high
vapor pressure, cavitation/boiling conditions in the pump suction are created.
The pump then loses suction hydraulics; to re-establish operations, the suction
must be re-established or primed, which can be time-consuming and not always
facilitate smooth stripping operation, the valve with high turndown (TABLE 3) will gently
open the minimum flow control valve when the discharge valve gets throttled by
the operator, and only 60 m3/hr at 10% travel is spilled back to the
pump suction, resulting in a smooth decrease in forward flow from 630 m3/hr
to 570 m3/hr. Throttling the discharge valve at the lowest speed gently
slows the stripping rate and ensures suction hydraulic flow is maintained and
reduced in a stable manner.
minimum flow control valve with low turndown (TABLE 2) will immediately let 200 m3/hr
flow through the recycle at 10% travel, creating a sudden disturbance to the
process dynamics and a potential risk of destabilizing the stripping operation.
The resulting forward flow suddenly equals 430 m3/hr in this case,
which can be too disruptive for the delicate stripping process.
a high-turndown minimum flow control valve has operational benefits during the
stripping operation. Requesting turndown so that operations can slowly reduce
the forward flow during the stripping process reduced the risk of unstable and
Turndown comparison summary. The behavior of the high-turndown and low-turndown valves in these two
scenarios are depicted in FIG.
3. Cv01 depicts the flowrates for the low turndown valve and Cv02
for the high-turndown valve.
The high turndown equals more
cost myth. The process engineer specifies the required
turndown requirements and conditions. For example, the design flow conditions can
be: a normal (minimum) flow control point of 630 m3/hr, then a maximum
flow of 10% more, and the minimum flow that needs to be controlled at 10% of the
630 m3/hr. Through the initial flow coefficient calculations (see TABLE 1), the
expected required turndown of the flow control valve can be determined.
these specifications, the flow control valve vendor must provide a valve with a
high turndown. In addition to this flow information, notes and specifications
on the datasheet are typically added, providing information on:
general, when turndown is within the valve control rangeability, a typical
valve size can be used without cost impact when specifying a high turndown. Rangeability
is the ratio of minimum and maximum controllable flow and is exclusive to the
inherent valve characteristics. The main contributors to an increase in the
cost of control valves are size, the material of construction and the valve’s pressure
control valve vendors classify small valves as < 4 in., mid-range control
valves as 4 in.–12
in., and large valves as ≥
selecting a control valve, vendors usually start small—maybe even half of the process line size
for smaller lines—or
will use the rule of thumb to have a control valve 2 in. smaller than the line
size for larger valves. The vendor will increase the valve size only if the
control range is not fulfilling the required process conditions. The cost
difference between valve sizes is roughly ±15%, given that the material of construction and
class ratings are the same. There are many types of minimum flow control valves
(e.g., globe, ball and butterfly valves) depending on the application. The same
is valid for the design of the valve’s internals. FIG. 4 depicts a minimum flow control
valve with guided perforated cage internals.
the specific turndown requirements are within a typical control valve’s range,
then specifying more turndown has no direct cost impact.
Matching the resistance curve and
pump performance map. For variable-speed pumps,
the datasheet should contain multiple cases, minimum speed, normal speed and
maximum speed. At each of these speeds, the variable-speed pump will have a
minimum flow requirement. This requirement is dictated by the variable-pump
speed vendor curve, which can be plotted against the system resistance curve as
a function of flowrate.
addition to the performance curves as a function of pump speed, two additional
lines depict a potential operational run of the pump ramping up to a certain
speed. This is in combination with the opening of the discharge valve, ramping
down to lower speeds and closure of the discharge valve, shown in FIG. 5.
specify the control valve, the minimum flow at minimum and maximum speed for
the variable-speed drive pump are required, and the minimum turndown and
maximum flow must be specified. This is summarized in TABLE 4, which forms
part of the datasheet by engineering.
depict the control points at minimum turndown and maximum opening, the vendor can
be asked for the % open Cv vs. the % travel characteristics. In this case, a
graph can depict the operating and controlling points for minimum and maximum valve
typical modified equal percentage control valve characteristic is depicted in FIG. 6, which shows
the maximum and minimum operating points. These points are not specific
to maximum and minimum speed, since (due to the formula of the flow coefficient)
the Cv at minimum speed can be the same as the Cv at maximum speed.
objective of the minimum flow control valve is to remove the mechanical energy
in the form of a pressure increase by the pump on recycle. The pump operates at
the minimum flow and puts in the corresponding differential head at that flow.
The flow control valve then takes out that differential head; therefore, the Cv
value is invariable to density since the density cancels out in the denominator
of the formula.
pressure drop in the recycle loop does affect the Cv value since the friction
factor is a function of the Reynolds number, and this number depends on the
kinematic viscosity. TABLE
4 depicts the influence of kinematic viscosity on the available
pressure drop for the minimum flow control valve. At higher viscosities, the
Reynolds number is lower; the friction factor is higher and, therefore, available
pressure drop for the valve is lower. This results in marginally higher Cv
values, as the recycle loop is a small system.
TABLE 4 indicates specified flow conditions at minimum and maximum speed, where
the Cv value at minimum flow at minimum speed equals the minimum flow at
maximum speed. As per the formulas in TABLE 1 where Cv = Qv √(ρ/∆P), it can be demonstrated that at low pump
speed—and thus low
pressure drop and flow over the control valve—the Cv value can equal the one at
high pump speed and high pressure drop and flow (FIG. 6).
Technical and commercial evaluation. Once the
process conditions and the turndown requirements have been specified on the
datasheet, the purchase order requisition will be prepared by the procurement
department and sent to approved vendors. Upon receiving information from each
vendor, a technical evaluation is prepared to determine the most favorable vendor
from a technical point of view.
variable-speed driven pumps have a minimum flow control line and not a minimum
flow point like fixed-speed pumps, an algorithm was be developed to determine
the minimum flow as a function of pump speed in revolutions per minute (rpm). Using
this algorithm, the most usage is made of the variable-speed pump’s performance
map (FIG. 2).
section explains how this algorithm was developed and implemented in the PMCS
for the minimum flow controller and for the instrumented protective function in
the ESD system that trips the pump based on low-low flow.
Process control objectives. The
objectives of the product pump variable-speed control are:
controller variable frequency drive (VFD) will (via its output) manipulate the
pump speed from its respective minimum speed to its maximum speed. Through the
variable-speed controller, the operator of the hydrocarbon terminal can set the
desired process conditions for the required operation.
The objective of the
minimum flow control is:
The minimum flow
controller will open the minimum flow control valve from the discharge to the
product pump to the suction if the flow tends to fall below the minimum
continuous stable flow value to ensure sufficient flow through the pump.
The minimum flow
control valve is fully open when the pump is not running. Confirming the valve is
open via a limit switch is required for pump startup. When starting, the
minimum flow controller ensures the pump is running above the minimum
continuous stable flow. When the forward flow increases, the minimum flow
controller will start closing the minimum flow control valve.
Integrating the minimum flow
control and variable-speed pump performance map. The
pump vendor’s performance map of the pump was used to determine the minimum
flow control line via a simple linear formula (Eq. 1). The formula uses the
process parameter pump speed in returns per minute (rpm) in this formula to
determine the setpoint of the minimum flow controller (min –flow.sp):
f(min_flow.sp) = a × rpm + b (1)
The calculated minimum
continuous stable flow setpoint has provided a margin on flowrate of some 10%
to control at a safe margin.
The minimum flow controller
is a protective controller and, therefore, not accessible to the operator
during normal operations. This is because manually controlling the minimum flow
controller setpoint or output can bring an operating pump below its required
minimum continuous stable flow, resulting in a subsequent loss of mechanical
However, when the pump is
not running, the operator can place the minimum flow controller in automatic or
manual mode, where the output to the control valve can be adjusted for
maintenance or other special operational purposes.
Minimum flow low-low flow trip. A low-low flow
trip has been implemented as ultimate pump protection, with its architecture
based on SIL classification. When the minimum flow control via the minimum flow
controller fails to maintain the minimum flow above the setpoint, a low-low
flow trip will stop the pump and immediately fully opens the minimum flow
control valve to safeguard the pump. Since the minimum flow of the pump is a
function of the pump speed, so is the low-low flow trip. A short adjustable
time delay is implemented in the control system to allow the pump to start,
since the starting flow is below the low-low flow trip. This time delay
prevents spurious trips of the pump on this low-low flow trip and allows the
pump to start.
The low-low flow trip
setpoint is set at a margin on flowrate of some 5% to trip the pump in a safe
and timely manner and fully open the minimum flow control valve. Once the pump
has started and the flow becomes higher than the minimum flow, the low-low flow
trip becomes active.
Overall process control scheme. The overall
process control implemented in the PMCS and instrumented protective
functionality in the ESD system are depicted in FIG. 7.
Takeaways. The article has demonstrated that:
SANDER P. B. LEMMERS is Head of the Project
Development Department in Dialog
E&C Sdn Bhd. He has more than 26 yr of
experience in both the technical and business facets of the global engineering,
procurement and construction industry. Lemmers is involved in the development
of small- and large-scale LNG and other liquefied gas and hydrocarbon terminals
in Malaysia and Southeast Asia. He holds a BSc degree and an MSc degree in
industrial engineering and management, and an MSc degree in chemical
engineering, from Twente University for Technical and Social Sciences in
Enschede, the Netherlands.
FONG HAU is a Process Engineer and has worked for Dialog
E&C Sdn Bhd for 9 yr. Her major experiences involve oil storage terminal
design and surge analysis. She earned her degree in chemical engineering from the
University of Technology Malaysia.
PUI CHEW TAN is
Manager, Instrumentation and Control, and has been working with Dialog E&C
Sdn Bhd for 9 yr. Prior to that, he had several roles as an electrical and instrument
maintenance engineer in a power station, a DCS and PLC programmer, and as an
application engineer with General Electric. Tan earned a degree in electrical engineering
from Melbourne University.