Advanced Process Control - Minimum flow control for centrifugal pumps

Summary

The 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

00:00

🔄 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.

05:01

🔌 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.

Keywords

💡Minimum Flow Control

Minimum Flow Control is a system designed to prevent damage to pumps by ensuring they operate above a specified minimum flow rate. It is central to the video's theme as it is the primary concept being explained. In the script, it is illustrated through an example where a pump with a capacity of 200 cubic meters per hour has a minimum flow rate of 100 cubic meters per hour, and recirculation is used to maintain this rate when actual flow drops below the minimum.

💡Pump Capacity

Pump Capacity refers to the maximum volume of fluid a pump can move in a given time, measured in the script as 200 cubic meters per hour. It is a fundamental concept in the video, as it sets the upper limit for the pump's operation and is used to contrast with the minimum flow rate required for safe operation.

💡Recirculation Line

A Recirculation Line is a pipe or conduit that returns a portion of the fluid back to the pump's inlet, ensuring the pump operates above its minimum flow rate. In the video's narrative, it is crucial for the function of the minimum flow control system, as it is the physical component that allows the pump to 'think' it is operating at the minimum required flow.

💡Flow Sensor

A Flow Sensor is a device that measures the rate of fluid flow and sends signals to control systems. In the script, it is used to monitor the flow rate at the pump outlet and trigger the recirculation valve when the flow drops below the minimum required, thus maintaining the pump's operation within safe parameters.

💡Control Valve

A Control Valve is a device used to regulate the flow of fluids through a pipeline. In the context of the video, it is part of the minimum flow control system, opening to allow recirculation when the flow sensor indicates that the flow rate has fallen below the pump's minimum requirement.

💡Centrifugal Pumps

Centrifugal Pumps are a type of pump that uses a rotating impeller to increase the velocity of a fluid, which then converts kinetic energy into pressure energy. The video discusses the need for minimum flow control specifically in relation to these pumps, as they can be damaged if they operate below a certain flow rate.

💡Horsepower

Horsepower is a unit of measurement for the rate at which work is done, indicating the power of the pump. In the script, it is used to categorize pumps and determine whether they require a minimum flow control loop, with different recommendations based on the horsepower rating.

💡Restriction Orifice

A Restriction Orifice is a small hole that limits the flow of fluid in a pipeline. In the video, it is suggested as a cost-effective alternative to a control valve for pumps with lower power ratings, ensuring a continuous minimum flow even when the pump's demand is higher than the minimum.

💡Parallel Operating Pumps

Parallel Operating Pumps refer to multiple pumps working together to provide a higher flow rate. The script discusses different options for providing minimum flow lines to such pumps, highlighting the challenges and considerations in ensuring each pump operates safely.

💡Minimum Spill Backpipe

A Minimum Spill Backpipe is a recirculation line that serves as a safety measure for pumps operating in parallel. The script explains that it can be used to maintain the minimum flow rate for each pump, ensuring that they do not operate below their safe limits.

💡Control System

A Control System in the context of the video refers to the mechanism that manages the operation of the minimum flow control, including the flow sensors and control valves. It is essential for the safe operation of the pumps, as it monitors and adjusts the flow to meet the minimum requirements.

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