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.