Tank storage is a key part of many processes. Storing water will ensure that there is enough water to fight a fire in a facility, and storing animal fats will ensure that there is the feedstock to make biodiesel or other finished products that require this as feedstock.
Keeping these tanks warm or above freezing temperatures is often just as important as the product stored in them. Having a tank that is properly designed to accommodate a heat source then is critical as well.
Several key design criteria should be followed when considering an electric heat source for a storage tank- such as tank design temperature, pressure, level control, heater elevation or location, and sensor placement.
Mechanical design
The mechanical design of storage tanks will require a specific temperature and pressure requirement. When designing a heater for that storage tank, it is vital to make sure that this heater can handle those pressures and create those temperatures.
Heaters such as flanged immersion heaters or pipe insert heaters are the most common method of electric heat for storage tanks. These units use ANSI flanges that are rated for pressure and temperature by their flange temperature and pressure rating.
Making sure that these heaters are properly rated to match your tank pressure and temperature is imperative.
Level control
Level control is one of the most commonly misdesigned components of a tank
Level control is one of the most commonly misdesigned components of a tank heating system when it comes to an overall good solid design. Float switches, ultrasonic and microwave, and even pressure sensors are effectively used in measuring not only the total tank level but multiple levels in that same tank.
The design and construction of storage tanks require that the facility knows how much product is in the given tank. Level measurements are often based on just measuring tank volume alone… how much “Stuff” is there to make more stuff? When an electric heater is used to warm or heat that product, it becomes critical to know at what elevation or height in the tank the heater is installed.
The product level
If the product level goes below the heater, that unit is then in the air instead of the product or content of the tank. This will cause the heater to add heat at a rate other than what it was designed for.
Level controls should override or shut off levels at the elevation or level of the heater so that the heater does not overheat. Similar designs are done for pumps so that they do not go dry. It is needed to interlock that level control for the heater to a heater shutdown loop.
Where in the tank the heater is installed is another important consideration. It is known that heat rises, so ideally the heat is to be applied at the very bottom of the tank.
Few specific things
Consideration should be given to a few specific things when determining how low one can go. Primary considerations include:
- Mounting flange diameter and location, tank by product or sludge, and removal space
- Mounting location- a flange or FNPT (Female National Pipe Thread) connections are most commonly used. Allowing for room to bolt on the studs and attach the heater is critical. Also allowing spacing on that same mounting flange for the thickness of the tank insulation- and still leaving enough space to get in there and bolt on the heater are a couple of good things to remember. One should get it as low as they can go, but leave room to work on it. Finally- when the heater is pulled out after the whole tank is installed and commissioned, will one bump into another tank or maybe a retaining wall? If one is low in the tank, that may change the type of heater being used or where it is located.
- Does the product being put into the tank have waste solids that settle out into sludge at the bottom of the tank? If so, one should make sure that the heater is not so low that it is sitting in that sludge. Being surrounded by sludge or settled tank products will reduce the heat transfer effectiveness of the heater and cause premature failure.
Another key design
Another key design consideration is where to sense the tank temperature
Another key design consideration is where to sense the tank temperature. Much like level control, designers often will require a temperature indicator or TI. When tank product temperature is critical, they want to always know they are ok on that front. But where that sensor is placed is vital.
Placing a TI up high in that designed tank will render it useless as soon as the product from the tank is pulled out. Putting it too low may allow it to be buried in sludge or gunk and years go on. When an electric heater of any kind is used, it is now known that when the tank level passes below the primary heat source, the heater can overheat and possibly damage the tank product. Ideally, a tank temperature sensor should be also placed at the level of the heat source.
If, for example, a heater is installed at 2 ft above the bottom of a tank, the TI should also be placed at that same elevation, 180 degrees from the heater (on the opposite side of the tank) This will allow the heater to create the heat that it needs without being shut down too quickly.
Key temperature sensing application
Another key temperature-sensing application is the heater surface or sheath high-limit shutdown sensor. If you have concerns about tank content damage in a runaway condition, and proper level control is installed, it’s wise to have a backup safety limit in the form of a high temp limit shutdown, “Temperature Switch High” or TSH as indicated on a P&ID.
This sensor is often placed right on the heater sheath. Common types of TSHs are Type K thermocouples since they can endure up to 220 F. This sensor is attached to an FM-approved safety limit controller whose duty it is to only watch for high temp runaway conditions.
This high-limit TSH function not only protects the product in the tank but also protects the mechanical design integrity of your tank since it is limited to a specific temperature.
Other Design Considerations
Heater sheath temperature is another important consideration for designing
Heater sheath temperature is another important consideration when designing- if the tank process temperature will cause the heater sheath to be hotter than the product can handle, coking or product damage can occur. For example, there is a tank that needs to be maintained at 100F.
The product in the tank will become damaged at 110F. The design requirements state that it is needed to heat the product from 50 to 100F. The tank pressure is 50 psi and the tank design temperature is 200F.
When the heater is installed and correctly controlled, that heater will need to be well above 110 F in order to heat that product up to 100F. Sometimes 100 to 150F above the process temperature. The product will get to the temperature, but it will be damaged in the process.
