Steam boiler feed tank is one the most important parts of any steam boiler room setup. Feed tanks are used to hold and pre-heat treated water (also called feed water) before entering a boiler. Feed tanks are sized to supply water uninterruptedly for a minimum of 10 minutes. Water Treatment Chemicals are injected into feed tank to act as Oxygen Scavengers. Steam is also injected into feed tanks to pre-heat the treated water to a temp level between 160-180F. The higher the water temperature the less the dissolved oxygen (DO) percentage in the water. It is crucial to reduce the amount of DO into the feed water to protect steam boilers from corrosion.

Steam Injection valves:

Almost all feed tanks are required to have a steam injection valve to maintain water temperature. There are different types of steam injection valves used to inject steam into feed tanks, here below are a couple of types widely used in the industry:

1- Self Acting control valves, these valves come with a temperature pilot with a capillary tube and a sensor to sense the feed water temperature in the tank and adjust the amount of injected steam accordingly. The cost of these valves is relatively low.

2- Control valves with electro/pneumatic positioners. These valves provide more accurate temperature control as they receive the control signal from a PID controller. The cost of these valves is relatively higher.

It is highly recommended to inject low pressure steam into feed tanks (less than 15psi) to avoid hammering and high noise levels.

How to size feed water tanks:

The rule of thumb is 1 Gallon for 1 BHP.

Here is a simple equation to calculate the required steam boiler feed tank size:

Boiler output (Lb/h) / 8.3 (Lb/Gallon) / 60 (min/hr) x 10 min (storage time) x 1.4 safety factor = Feed Tank size in Gallons

Feed tanks come mainly in stainless steel and carbon steel. The later has a lower initial cost but the stainless steel one lasts longer.

Level Controller

Feed tanks come with different level controllers to maintain water level. Below is a list of common level controllers types used in the industry:

1- Probes , usually this setup comes with 3 probes (valve open, valve close and common probe). This setup is simple and easy to maintain with no moving parts.

2- Float switch, this setup comes with 2 float switches to open and close the makeup valve.

3- DPT (differential pressure transmitter), this setup comes with pressure differential sensor that measures the water level in the tank and sends signal to a modulating makeup valve to maintain the water level. The cost of this setup is higher than the above 2 options.