tanks ( type 1-3)
The functional layout is illustrated in Diagrams 23 and 24.
In this paragraph, we will analyse the dimensioning of the main circuit components.
Storage tank
The storage volume should be determined in relation to the fuel oil delivery method and derives from a compromise between the supply transport cost, delivery guarantee and installation cost for the tank.
As a general indication, the following minimum types can be considered:
• two tanks of 45,000 kg;
• three tanks of 25,000 kg;
Pumping unit P1 (transfer ring)
This component is only present in plant engineering types with service tanks.
This pumping unit, denominated transfer, must have a capacity equal to 1.2-1.5 times the peak maximum consumption, and comprise a pair of pumps and filters with the possibility of switchover in by-pass. They are :
m i = maximum fuel consumption of the Nth burner;
Mi = pump delivery of the Nth burner;
The pumping unit delivery is equal to:
Qp1 = 1,2÷1,5 · (∑mi) eq. 2.6.2 -14
This pumping unit must have a self-cleaning blade filter or similar, equipped with a heater, with meshes with a dimension between 400 and 600 µm.
Fuel oil pumps can be monobloc or with separate gear or screw motors. The number of revs is normally low (900÷1,400 g/1’), and as a rule the more viscous the oil, the lower the number of revs must be.
Pumping units already complete with filter, pumps, pressure regulating valve, gauge, check valve and shut off valve are available on the market.
The head ensured by these pumps normally ranges between 100.00÷600,000 Pa (1÷6 bar).
Service tank S
This component is only present in plant engineering types with service tank.
The service tank acts as a communication element between the transfer section and the ring section for final fuel oil pre-heating. This allows accumulating a certain amount of liquid fuel between the cistern and the burner. The tank must have the following characteristics:
• tank capacity equal to 2-3 times the sum of the maximum hourly drawing capacities of the burners:
VS= 2÷3 · (Mi) · 1 [kg] eq. 2.6.2 -15
• entry of the fuel oil from the end plate;
• double fluid/electrical pre-heater; the fluid pre-heater (warm water or vapour) to be positioned immediately above the arrival point for the liquid fuel; the electrical pre-heater above the fluid pre-heater with integration and emergency functions;
• drawing off the liquid fuel above the pre-heaters.
The tank must be equipped with the following devices:
• end plate outlet for water and sediment;
• level control with minimum and maximum alarm equipped with self-checking systems;
• atmosphere breather pipe;
• "over full" device with return line to storage tank;
Pumping unit P2 (main ring)
This pumping unit must have a delivery equal to at least 3 times the sum of the maximum drawing capacities of the burners and comprise a couple of pumps and filters with the possibility of switchover in by-pass:
Qp2= 3 · (Mi) eq. 2.6.2 -16 This over-dimensioning is due to the need to maintain a stable pressure independently from the possible combinations of the various burner running stages.
This pumping unit must have a self-cleaning blade filter or similar, equipped with heater and with meshes measuring between 200 and 300 µm.
The head of the pumping unit should be calculated on the basis of the ring pressure to be guaranteed, as a rule greater than 100,000 Pa (1 bar), and the pipeline pressure drops calculated as specified below.
Remember that in the absence of definite information from the burner manufacturer
concerning the pump delivery on board the machine, the following characteristic values can be used:
• for multi-stage burners: M = 1.3÷1.5 ·m
• for modulating burners: M = 2.0÷2.5 ·m Pressure regulating valves
The pressure regulating valves are required to maintain the pressure in specific parts of the circuit and therefore the delivery desired. They are installed on the main ring and essentially comprise a valve body in cast iron with hydraulic couplings for high and low pressure and a by-pass regulator piston with a related spring and rating organ.
Their function is such that, even under large delivery variations, the required pressure is maintained within a certain tolerance range.
The choice of these valves is made on the basis of project data:
• delivery equal to that of the pumping unit in the related circuit;
• pressure range between 50,000 Pa (0.5 bar) and 500,000 Pa (5 bar), typically between 100,000 and 400,000 Pa (1÷4 bar).
Calculating the pipelines
The pressure drops in the pipelines are the sum of those distributed along the pipeline and those concentrated due to connecting elements and hydraulic accessories (filters, valves, etc…).
To correctly dimension the pipelines the following sizes are defined:
LEFF = effective pipeline length [m];
LEQUIV = sum of the equivalent lengths relative to concentrated pressure drops as a result of the connecting elements and hydraulic accessories [m];
LTOT = total pipeline length, sum of the effective and equivalent lengths [m]:
LTOT= LEFF + LEQUIV [m] eq. 2.6.2 -17 The equivalent lengths relative to the concentrated elements of the components must be gained from the manufacturer's technical specifications. If these values are not available, some tables exist, as illustrated in section 5, which contain the equivalent lengths referring to the main concentrated resistances.
Any filters must be calculated with the effective head loss procured by their presence. If the exact pressure drop value is not available, it is possible to assimilate the filter to an open valve.
The following procedure can be used both for Service tank
Diagram 63
A – oil from reservoir B – return oil from the burner D – tank drainage
L – tank gage M – oil to the burner R – electrical heater V – steam heater S – overflow discharge
the transfer and the primary circuits.
To correctly dimension the pipelines, the system must be divided into two parts:
• intake pipelines;
• delivery pipelines.
This division is justified by the fact that while the pump performances on the delivery pipeline do not create any problems given the high heads that can be achieved, in the range of 300,000÷500,000 Pa (3÷5 bar), on the intake pipeline there are some maximum depression limits which must not be exceeded to avoid gasification problems with the fuel oil with consequent problems of pump cavitation.
This value (NPSH) is supplied by the pump manufacturer and in any case cannot be any lower than 50,000 Pa (0.5 bar).
The intake pipeline is dimensioned in relation to the following parameters:
• maximum project-related pressure drop (depression) ∆Pprog [Pa];
• minimum speed Vmin equal to 0.15 m/s;
• minimum internal diameter Dmin no less than 0.008 m
The maximum project-related pressure drop is equal to:
∆Pprog =∆Pamn -∆hasp - ∆Pacc [Pa]
eq. 2.6.2 -18 where:
∆Pamn = the absolute pressure allowed at intake (NPSH) indicated by the pump manufacturer; otherwise, this pressure must not be less than 50,660 Pa (0.5 bar);
∆hasp = intake height;
∆Pacc = head loss due to the presence of any hydraulic accessories not calculated in the determining the equivalent lengths present on the intake pipeline (filters, etc…)
The intake height is equal to:
∆hasp = ∆hgeom .ρ. 9,81 [Pa] eq. 2.6.2 -19 where:
∆hgeom = the difference in height between the fuel test point in the tank and the centre of the delivery pump [m];
ρ = heavy fuel oil volume mass [kg/m3];
The value of ∆hgeomis positive if the tank test point is lower than the centre of the pump, negative if the tank test point is higher than the centre of the pump.
The liquid fuel volume mass depends on the temperature according to the following formula:
eq. 2.6.2 -20 where:
ρ= liquid fuel volume mass [kg/m3];
ρ15 = liquid fuel volume mass at the reference temperature of 15°C equal to 990 kg/m3; t = liquid fuel transfer temperature [°C];
β= expansion formula equal to 0.00063°C-1;
The liquid fuel transfer temperature is determined with reference to the constructive limits of the pumping unit, which in fact determine the maximum viscosity of the liquid that can be pumped and its temperature. As a general rule, the viscosity limit ranges between 30°E and 50 °E (228÷380 cSt) which determines a liquid fuel transfer temperature around 50÷60°C. There are some pumping units capable of pumping liquids with a viscosity greater than 100°E. In any event, it is good practice to keep the viscosity values below 50°E.
The pipelines are dimensioned according to the following formula:
eq. 2.6.2 -21 where:
d = internal pipeline diameter [m];
γ = kinematic viscosity of the liquid fuel transfer temperature [m2/s];
LTOT = total pipeline length, sum of the effective and equivalent lengths [m];
m = mass-related delivery of the pumping unit [kg/s];
∆Pprog = maximum project-related pressure drop (depression) [Pa];
It should be pointed out that, in technical practice kinematic viscosity is expressed either in cSt or in a unit of measure depending on the type of viscometer used to measure the viscosity (Engler, Saybolt universal, Redwood, etc…); therefore, before using the previous formula the kinematic viscosity must be transformed into cSt using the tables and alignment charts indicated in section 5, remembering that:
1 cSt = 1 mm2/s = 10-6m2/s eq. 2.6.2 -22 To determine the minimum internal diameter of the pipeline, the total length of the pipeline
d= 42 . γ . LTOT .m
∆Pprog
√
ρ= ρ15 1 + β .(t - 15)
must be determined and consequently the equivalent length that, in turn, depends on the internal diameter of the pipeline. We must therefore presume an initial provisional diameter for estimating the equivalent lengths.
An initial diameter estimate can be made by presuming a liquid fuel flow speed, pre-determining the diameter with the following formula:
eq. 2.6.2 -23 where:
d = internal pipeline diameter [m];
Q = liquid fuel delivery in volume [m3/s];
V = liquid fuel flow speed as equal to 0.15÷0.20 m/s;
Once the equivalent length and, consequently, the total length have been determined, it is possible to use the equation (2.6.2-21) to determine the minimum internal diameter of the pipeline.
If the diameter calculated in this manner is significantly different to that presumed for calculating the equivalent lengths, the equivalent lengths must be re-calculated with the new diameter using the equation (2.6.2-23) and subsequently repeat the diameter calculation using the equation 2.6.2-21.
The pipeline will correspond to the commercially available diameter immediately above that determined using the equation (2.6.2-21).
At this point, the speed in the pipeline must be checked using the following formula:
[m/s] eq. 2.6.2 -24
where:
d = internal pipeline diameter [m];
Q = liquid fuel delivery in volume [m3/s];
If the transfer speed is lower than the limit value of 0.15 m/s, proceed as follows:
• the pipeline diameter that guarantees this certain minimum speed should be chosen using the formula:
eq. 2.6.2 -25
• the total maximum pipeline length (effective + equivalent) connecting the tank and the pump is determined so as not to
A= Q ⇒ π . = d
exceed the project pressure drop using the following formula:
eq. 2.6.2 -26
The pump will be connected at a distance from the tank, which should not exceed LTOT considered as the sum of the effective and equivalent lengths.
If the resulting diameter were less than 0.008 m, a pipeline with an internal diameter of 0.008 m should be chosen taking care to up-rate the pump delivery so that the fluid speed is greater than 0.15 m/s.
As far as the delivery pipeline is concerned, the diameter should be chosen in relation to the maximum allowed speed equating to 0.6 m/s using the equation (2.6.2-23), more precisely:
eq. 2.6.2 -27 where:
d = internal pipeline diameter [m];
Q = liquid fuel delivery in terms of volume [m3/s];
V = liquid fuel flow speed equal to 0.6 m/s;
The pipeline will correspond to the commercially available diameter immediately above that is determined using the equation (2.6.2-23). After which, proceed calculating the pressure drop of the entire circuit (transfer or primary ring) using the following equation:
eq. 2.6.2 -28
where:
d = internal pipeline diameter [m];
γ = kinematic viscosity at the liquid fuel transfer temperature [m2/s];
LTOT = total pipeline length, sum of the effective and equivalent lengths [m];
m = mass-related delivery of the pumping unit [kg/s];
∆Pprog = calculation pressure drop [Pa];
The calculation pressure drop must be added to the loss due to any hydraulic accessories (filters, etc…) present on the delivery pipeline, the loss present on the delivery pipeline and
∆Pprog= 42.γ . LTOT.m d4
A= QV ⇒ π .d42 = QV ⇒ d=
√
4 . Qπ . VLTOT= d4 . ∆Pprog 42 . γ .m
the difference in height between the intake pipeline and the delivery pipeline:
eq. 2.6.2 -29 where:
∆Ptot = total pressure drop [Pa];
∆Pcalc = calculation pressure drop of the delivery pipeline [Pa];
∆Pacc = head loss due to the presence of any hydraulic accessories not calculated in determining the equivalent length, present on the delivery pipeline (filters, etc…) [Pa];
∆Hpipelines = difference in height between the intake pipeline and the delivery pipeline