Advanced Process Control - Minimum flow control for centrifugal pumps
Summary
TLDRThe script explains the concept of minimum flow control in centrifugal pumps, using a pump with a 200 cubic meters per hour capacity and a 100 cubic meters per hour minimum flow rate as an example. It details how a recirculation line can maintain the pump's minimum flow requirement, preventing damage. The necessity of minimum flow control varies with pump size and power, with larger pumps often requiring more sophisticated control systems. The script also discusses different control arrangements for pumps in parallel operation, highlighting the trade-offs between cost and effectiveness.
Takeaways
- 🚰 The concept of minimum flow control is essential to prevent damage to pumps by ensuring they operate above their minimum flow rate.
- 🔄 A recirculation line can be used to maintain the minimum flow rate when the actual flow into the pump is insufficient.
- 📶 A flow sensor on the pump outlet sends a signal to the control valve on the recirculation line to adjust the flow rate as needed.
- 🔩 The recirculation loop trick does not increase the overall flow in the piping system but ensures the pump's minimum flow requirement is met.
- 🚫 Not all centrifugal pumps require a minimum flow control loop, especially small pumps that are less expensive.
- 🏗️ For larger pumps, especially those with power over 5 horsepower, a minimum flow control loop is often necessary to prevent operational issues.
- 🔧 The rule of thumb for minimum flow control is no line for pumps under 5 horsepower, a continuous line for those between 5 and 20 horsepower, and an on/off line for pumps between 10 to 30 horsepower.
- 💡 Pumps over 35 horsepower typically require a minimum flow line with a control valve, which is the most common and sophisticated method.
- ⚙️ There are two common arrangements for minimum flow control: a flow loop and a pressure loop, with the flow loop being more commonly selected.
- 🔄 When multiple pumps operate in parallel, different options for minimum flow lines exist, with a shared or dedicated recirculation pipe being the most practical.
- 📊 The choice of control system for the minimum flow pipe can impact the ability to detect and address issues with individual pumps in a parallel arrangement.
Q & A
What is the purpose of minimum flow control in pumps?
-The purpose of minimum flow control is to prevent damage to the pump by ensuring that the flow rate does not drop below a specified minimum, which is necessary for the pump to operate safely and efficiently.
What is the minimum flow rate for the pump in the example provided?
-The minimum flow rate for the pump in the example is 100 cubic meters per hour, as specified by the vendor.
How does a recirculation line help in maintaining the minimum flow rate of a pump?
-A recirculation line helps by allowing excess flow to be redirected back to the pump's inlet when the flow rate drops below the minimum required, thus maintaining the pump's operation within safe parameters.
What happens if the flow rate into the pump is 165 cubic meters per hour?
-If the flow rate is 165 cubic meters per hour, the pump operates normally, and the sensor on the pump outlet sends a signal to keep the recirculation valve closed, as the actual flow is above the minimum required.
What action is taken when the flow rate drops below the minimum required, such as to 80 cubic meters per hour?
-When the flow rate drops below the minimum, the flow sensor signals the controller to open the recirculation valve partially to recirculate the required amount to meet the minimum flow rate, in this case, 20 cubic meters per hour.
Does the recirculation loop increase the overall flow in the upstream and downstream piping system?
-No, the recirculation loop only increases the flow rate within the loop itself to a number higher than the minimum required. The overall flow in the upstream and downstream piping system remains unchanged.
Are minimum flow control loops necessary for all centrifugal pumps?
-No, minimum flow control loops are not necessary for all centrifugal pumps. Small pumps of less than five horsepower and pumps in circulating closed systems may not require them.
What is the rule of thumb for providing minimum flow control for pumps with different power ratings?
-For pumps with power lower than five horsepower, no minimum flow line is required. Between 5 and 10-20 horsepower, a continuous minimum flow line with a restriction orifice is suggested. For 10 to 30 horsepower, an on-off minimum flow line is needed, and for pumps above 35 horsepower, a minimum flow line with a control valve is necessary.
What are the different arrangements for controlling minimum flow in centrifugal pumps?
-There are at least two different arrangements: a flow loop controlled by a flow sensor and a pressure loop controlled by a pressure sensor. The flow loop is usually selected.
How should minimum flow lines be provided for parallel operating pumps?
-There are three options: a shared minimum recirculation pipe for two operating pumps, dedicated recirculation pipes that merge downstream before reaching the reservoir, or fully dedicated minimum flow pipes for each pump. Option 2 is often chosen as a balance between cost and effectiveness.
What are the issues with using total flow for control in a system with parallel operating pumps?
-The control system cannot recognize which pump is suffering from flow lower than the minimum flow, as the total flow may still appear adequate even if one pump is not receiving enough flow.
Outlines
🔄 Understanding Minimum Flow Control
This paragraph introduces the concept of minimum flow control in pumps. It uses an example of a pump with a capacity of 200 cubic meters per hour and a minimum flow rate of 100 cubic meters per hour. A recirculation line is suggested to maintain the pump's minimum flow rate. The paragraph explains how a sensor on the pump outlet interacts with a control valve on the recirculation line to ensure the pump operates above its minimum flow rate. It also clarifies that this method only affects the flow within the recirculation loop and not the overall system flow. The discussion then moves to when minimum flow control is necessary, mentioning that it's not required for all centrifugal pumps, especially small ones or those in closed circulating systems. However, larger pumps and those on the mainstream may require it. The paragraph concludes with guidelines on when to use minimum flow control based on pump power.
🔌 Options for Minimum Flow Control in Parallel Pumps
The second paragraph delves into the specifics of minimum flow control for pumps operating in parallel. It presents three options for providing minimum flow lines: a shared recirculation pipe for two pumps, dedicated recirculation pipes that merge downstream, and fully dedicated minimum flow pipes for each pump. The paragraph discusses the pros and cons of each option, highlighting the potential issues with sensor accuracy and the cost implications. It suggests that most companies opt for the second option due to its balance between cost and effectiveness. The paragraph also touches on the two types of control systems that can be applied to the minimum flow pipe: one that uses total flow for control and another that uses minimum flow. It points out the limitations of each system, such as the inability to identify which pump is underperforming or to provide adequate flow to both pumps if they both require additional flow.
Mindmap
Keywords
💡Minimum Flow Control
💡Pump Capacity
💡Recirculation Line
💡Flow Sensor
💡Control Valve
💡Centrifugal Pumps
💡Horsepower
💡Restriction Orifice
💡Parallel Operating Pumps
💡Minimum Spill Backpipe
💡Control System
Highlights
The concept of minimum flow control is crucial for pump operation.
Pump capacity is 200 cubic meters per hour with a minimum flow rate of 100 cubic meters per hour.
A recirculation line is used to maintain the minimum flow rate.
When flow is above minimum, the control valve remains closed.
Below minimum flow, the control valve opens to recirculate flow.
Recirculation loop tricks the pump into thinking the flow is at minimum.
The overall flow in the piping system remains unchanged despite recirculation.
Sensor placement is critical for effective minimum flow control.
Not all centrifugal pumps require a minimum flow control loop.
Small pumps under five horsepower typically do not need minimum flow control.
Pumps in circulating closed systems may not require minimum flow control.
Large pumps and those on the mainstream may require minimum flow control.
A rule of thumb for providing minimum flow control based on pump power.
For pumps under 5 horsepower, no minimum flow line is required.
Pumps between 5 and 20 horsepower may use a restriction orifice instead of a control valve.
Pumps between 10 and 30 horsepower need an on-off minimum flow line.
Pumps over 35 horsepower require a control valve for minimum flow control.
Two common arrangements for controlling minimum flow: flow loop and pressure loop.
Flow loop is usually preferred over pressure loop for minimum flow control.
Options for providing minimum flow lines for parallel operating pumps.
Option 2 with merged recirculation pipes is a common choice for parallel pumps.
Two types of control systems can be applied on the minimum flow pipe.
The total flow control system may not identify individual pump issues.
The minimum flow control system can't satisfy both pumps if they both suffer from low flow.
Transcripts
the concept of minimum flow control is
shown in the following example
let's assume that this pump has a
capacity of 200 cubic meters per hour
the vendor specified that the minimum
flow rate of the pump is 100 cubic
meters per hour
we can provide a recirculation line from
the outlet of the pump back into the
inlet line
if the flow rate into the pump is 165
cubic meters per hour the pump is happy
since its actual flow is higher than the
minimum flow
in this case the sensor on the pump
outlet sends a signal to the control
valve on the recirculation line and it
remains closed
however if the flow drops below 100
cubic meters per hour let's say for
example 80 cubic meters per hour then
the flow sensor on the outlet will send
a signal to the controller to say hey
i'm short of my minimum required flow by
20 cubic meters per hour and i'm worried
about the pump
please open the valve enough to
recirculate 20 cubic meters per hour so
we can fool the pump into thinking that
the flow is 100 cubic meters per hour
and make it happy
so the control valve on the
recirculation line would be partially
open to provide the required flow of 20
cubic meters per hour
this flow is sent back to the inlet to
satisfy the minimum flow condition of
100 cubic meters per hour and thus
prevent damage to the pump
this is the concept of minimum flow
control
now it is important to recognize here
that this trick only increases the flow
rate inside the recirculation loop to a
number higher than 100 cubic meters per
hour to fool the pump
we are not able to increase the overall
flow in the whole upstream and
downstream piping system
the flow in those pipes is still 80
cubic meters per hour
the point here is that the sensor should
be placed as close as possible to the
pump and within the recirculation loop
now the question is whether we need a
minimum flow control loop for all
centrifugal pumps or not
the answer is no
you will not see this type of control on
each centrifugal pump at your facility
we don't need minimum flow control loop
for all centrifugal pumps
the following examples of pumps may not
need a minimum flow control loop
small pumps of less than five horsepower
may need it but since they are not
expensive we don't bother to put an
expensive minimum flow control loop on
them
another example is pumps in a
circulating closed
they may not need a minimum flow control
loop because the flow in such pumps is
fairly constant
however the following examples of pumps
may require minimum flow control
for example large pumps of five
horsepower or more and pumps on the
mainstream
the rule of thumb for providing minimum
flow control is as follows
for pumps with power lower than five
horsepower no minimum flow line is
required
when the power of the pump is between 5
horsepower and 10 to 20 horsepower you
need a continuous minimum flow line
as depicted in this process arrangement
instead of going to the expense of
installing a control valve we can simply
put a restriction orifice on this line
by doing so we are always re-circulating
a portion of flow even in cases where
flow to the pump is higher than the
minimum flow
when the flow to the pump is higher than
the minimum flow we don't really need
recirculation but because of the
installed orifice we are continuously
wasting energy and we know that but the
pump is so small that installing an
expensive control loop for it is hard to
justify
for pumps with power between 10 to 30
horsepower you need an on off minimum
flow line
this is a cheaper option than installing
a control valve
in this case the flow control loop will
just provide an on off function
finally for pumps with power higher than
35 horsepower you definitely need a
minimum flow line with a control valve
this is the most complicated most
expensive option but it is the most
common method of controlling minimum
flow in centrifugal pumps
this type of control can be done with at
least two different arrangements
with a flow loop as seen
here or with a pressure loop
we have the option of controlling by
flow sensor or by pressure sensor
usually the flow loop is selected
now when there are two or more pumps in
parallel as highlighted here and only
one of them is operating and the rest
are spares
a minimum spill backpipe from the common
header works well
the question is what the minimum flow
should be when there are parallel
operating pumps
we have three options for providing
minimum flow lines for parallel
operating pumps
in option one a shared minimum
recirculation pipe is provided for two
operating pumps as highlighted here
in option 2 dedicated recirculation
pipes are provided at the beginning but
they are merged downstream before
getting to the upstream reservoir
then option 3 fully dedicated minimum
flow pipes are provided for each pump in
parallel arrangement and both pumps are
operating
the problem with option one is that the
flow from the two pumps compensate for
each other and the sensor cannot tell if
one pump is starving or not
the sensor could be telling you that the
system is okay but one of the pumps
could actually be in bad shape
option three may be the best system but
it is the most expensive option
for this reason most companies choose
option 2 and it is most likely that this
arrangement is implemented at your
facility
in all cases at least two different
types of control system can be applied
on the minimum flow pipe as shown in
this next figure
in the left hand schematic the total
flow is used for the purpose of control
while in the right hand schematic the
minimum flow is used
the issue with the left-hand schematic
is that the control system cannot
recognize which pump is suffering from
the flow lower than the minimum flow
the issue with the right-hand schematic
is that the control system cannot
provide the flow to satisfy both
suffering pumps if both pumps are
suffering from flow lower than minimum
flow but with uneven difference to the
minimum flow
you
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