Page 249 - Boiler Operator’s Handbook

234 Boiler Operator’s Handbook

properly; viscosity giving us an indication of its ability onto the pump set. A relief valve, not a safety valve.
to flow. Storage tanks should be heated only enough to Even when more precise pressure control is provided
get the oil to flow to the day tank or fuel oil heaters, any- you normally have relief valves at the pump set.
thing hotter is just a waste of heat. That’s because most The self contained relief valve has to experience
storage tanks are not insulated. Heating the oil to the a change in pressure to change the flow of oil. In order
right viscosity for burning should happen just before it to be stable in operation a reasonable pressure droop of
goes to the burners. ten pounds minimum is required between conditions of
It’s necessary to run some of that heavy oil through no fire and full load on all boilers. A relief valve might
the piping of an idle boiler to keep it flowing. We call return all the oil to the tank at a pressure of 180 psig and
that recirculation and it’s essential for oils that could close off that port so all oil flows to the burners at 170
become solid in the piping and prevent our starting the psig. If you try to install one with a smaller droop the
idle boiler. There’s normally one globe valve in the pip- flow and pressure will be unstable. It has to be that way
ing that returns the oil to the pump suction or the tank so don’t expect the pressure relief valves at the pump set
(return oil piping) and that valve is throttled for several to maintain a constant oil supply pressure.
reasons. If we open it too far it can return more oil than It’s the variation in supply pressure that makes for
the pump is delivering with a resulting drop in oil sup- tiny variations in flow through the fuel oil flow control
ply pressure. Carelessly open a recirculating valve too valves in certain burner systems so additional provi-
far and you can force a shutdown of the entire plant. sions for pressure control are usually provided in an oil
If you don’t recirculate the oil enough the heat loss- system. Since the pump set is usually remote from the
es in the piping will lower the temperature until the oil burners a second pressure adjustment is made closer
is too cold when it gets to the burner. You need to open to the burners by a back pressure regulator. It’s a self
the valve enough to get the hot oil to the burner. On the contained control valve that maintains a more constant
other hand, the oil can return to the day tank to raise its pressure on its inlet by dumping some of the supplied
temperature so high that the pumps can’t create enough oil into the fuel oil return line. It’s really a relief valve
pressure and you’re shutting the plant down again. That but normally has a much larger diaphragm so the
happens because more oil slips back through the pump swings in pressure are not as great as they are for the
as the viscosity increases and, therefore, less is forced pump set relief valve. The two in combination produce
out the piping to the heaters and burners. a much lower droop.
In many plants the operators aren’t trusted to do For really precise oil supply pressure control two
it right so the recirculating control valves (those globe measures are used. One is a pressure regulator at each
valves in the return piping) are set and locked or the boiler. The regulator has a large degree of droop but since
handwheels are removed so you can’t mess with them. it’s repeatable the pressure at any particular firing rate
The best of both worlds is to throttle the recirculating is the same regardless of oil supply pressure. The other
control valve enough to keep oil flowing to the burners is installation of a more elaborate back pressure control
and back the return line with only one boiler (the one system, from pilot operated valves to a complete control
that you would start up if necessary, sometimes called loop with transmitter, PID controller, and control valve.
the standby boiler) having enough recirculating flow to Heating of the heavy fuel oil on small systems
get the right temperature at the burner. That way flow may consist of a simple temperature actuated control
is assured but you’re not returning so much that the oil valve but most of the systems use a temperature piloted
entering the pumps gets too hot. pressure control valve. A valve that acts on tempera-
Almost all fuel oil pumps are positive displace- ture alone will allow large swings in oil temperature
ment pumps. Gear types and screw types for the most with swings in flow to the burner because the control
part, they’re capable of raising the pressure of the oil valve doesn’t know the oil flow has increased until the
considerably so it can be delivered to the burners at a colder oil gets to it. By then the lower steam pressure
pressure high enough for proper atomization. Since it’s has allowed the metal of the heat exchanger to cool as
a positive displacement pump it’ll deliver a relatively well so the temperature controller will have to over-
constant quantity of oil. The oil you don’t burn is re- react. A temperature piloted control valve simply uses
turned, sometimes to the pump suction, others to the the temperature of the oil to set a steam pressure to be
fuel oil day tank or storage tank. To maintain pump dis- maintained in the oil heater. When the oil flow increases
charge pressure and control the flow of oil to the tanks it will use more steam to heat it and the pressure in the
requires a relief valve, either pump mounted or piped heater will start to drop. The pressure controller opens