In smaller systems it is normal that one pump will be used for both pumping to the boiler and condensate return. Larger systems tend to require multiple pump banks that are dedicated to both processes.
Important – when selecting pumps for boilers ensure you receive information on:
- Volume metric boiler feed water rate or Boiler MCR (maximum continuous rating which gives you flow rate)
- Pump discharge pressure
- Inlet temperature of water
- NPSHA available
Method of control
There are two main methods of controlling a boiler.
1. Boiler on/off
The most cost-effective way is an on/off control that utilises floats or electrodes mounted directly in the boiler and, depending on the water level, will start and stop the feed pump. Anti-siphon valves are also used on this control method which prevents the boiler from flooding. Boilers that utilise this type of control are of a smaller design up to 300kW.
2. Continuous operation
A water tube boiler comprises a number of tubes surrounded by fire created by gas or oil. The feed pump passes water through the tubes. The pump runs continuously whilst the flow varies depending upon the demand of the boiler. The pump will tend to need some form of protection to prevent running beyond its maximum capacity. Normally, a bypass line is installed which is either left open to a tank or controlled via a modulating valve.
a. Modulation control
With this type of control the pump is either controlled via a control valve in the main pump discharge line with a bypass; an orifice plate is installed into the bypass or the pump is controlled by a modulating valve fitted in the bypass line. The signal for modulation can be provided by either a level, pressure or temperature controller. The advantage of this type of control method is a smaller pump with better efficiency and reduced running costs.
When using modulating control it is essential to install a bypass pipe line back to the condensate tank or deaerator. Do not return to pump suction.
b. Bypass control
Another method of control is to place a modulating control valve in the bypass line; this acts in the opposite way to the valve above. As the water level in the boiler rises the modulating valve in the pump bypass line will open and return water back to the condensate tank thus, reducing the water flow to the boiler. Normally a high-level alarm will activate if there is loss of boiler pressure.
NPSH
In all boiler applications, due to the use of the pump with higher water temperatures (often above 100°C), it is essential that the NPSH be calculated for the system as incorrect selection may result in the pump being damaged. It is not unusual for the height of the condensate tank or de-aerator to be set to ensure sufficient static head to provide the NPSHA.
Boiler manufacturers are acutely aware that their application can be extreme and that the pump may be operating constantly and at high temperatures. Problems are often attributed to the pump but when analysed, these problems are usually down to a system problem.
Ensuring you have the right pump for the right application involves a thorough set of considerations. Consult a knowledgeable pump supplier like Kelair Pumps to discuss your pumping requirements.
Article courtesy of Kelair Pumps Australia “When Pump Knowledge Matters” Phone 1300 789 466, or visit www.kelairpumps.com.au.
