We are often asked about the importance of pump flow control management and how it impacts the temperature control loop in Direct steam injection heating. Many fluid process heating applications such as hot water for plant sanitation, hot water for jacketed vessels, and boiler feedwater heating rely on a steady delivery of hot water at the target temperature set-point.
The steam injection heater relies on the fluid pump for supply of water. Any changes in water flowrate to the heater from the pump, will require the temperature control loop to monitor, so that the temperature controller can send the appropriate control signal change to steam injection heater.
It is important to avoid water flow changes that occur faster than the temperature control loop can respond. Steam injection heating is very responsive to control changes and, unlike indirect contact heaters such as heat exchangers, with a steam injection heater there is generally no lag between the process control calling for the addition of steam and the heater responding. A temperature control loop will have a lag in response until instruments register a change and the temp controller responds which can take up to 10 seconds. For example, if the flowrate drops 50%, the temperature rise will double. This can lead to spikes in temperature and steam flashing damaging steam injector components.
Pump flow changes should be ramped allowing the temperature control loop to respond. Note that rapid changes in flow rate will lead to continuous control loop adjustments, which can impact temperature control performance. Rapid drops in liquid flow can lead to steam flashing until the temperature control can respond.
Below are 5 tips of liquid pump control recommendations to optimize the heater and temperature control loop performance:
- Never stop the liquid pump before shutting down the heater.
- Avoid rapid changes in water flow rate to allow the temperature control loop time to respond.
- Monitor water flow upstream from the heater with a flow switch. If water flow is lost, the steam injection heater should be shut down first and then the steam isolation valve should be closed.
- Add a water Hi-temp alarm on the discharge side of the heater. If there is excessive temperature rise an alarm should trigger and a shutdown control signal can be sent to the heater.
- The heater can also be interlocked to pump operation where the heater will only operate if the pump is on.
The above should help you set-up a stable and reliable water heating system at your required water temperature. For additional information, please contact the team at ProSonix.
