Systems with pressurised ring If the burner pump cannot self-feed because

the distance and/or the difference in height of the tank are greater than the values supplied by the burner manufacturer, a pressurised ring circuit must be adopted.

The ring-type circuit comprises a pipeline, which departs from and returns to the storage tank, with an auxiliary pump to make the fuel flow under pressure.

Pumping unit P (main ring)

This pumping unit must have a delivery equal to at least twice 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 = 2 · (∑Mi) eq 2.6.2-1

where Mi is the pump delivery of the individual burner.

This over-dimensioning is necessary to guarantee stable pressure in the ring independently from the possible burner functioning combinations.

This pumping unit must have a line filter with a metal net cartridge for diesel oil, to separate the impurities and water that may be present in the fuel.

The pumping unit head should be calculated on the basis of the residual pressure which must be guaranteed to the ring and the pipeline pressure drops calculated as specified below.

In the absence of certain information from the burner manufacturer concerning the pump delivery on the machine, the following typical values can be used:

• for multi-stage burners: M = 1.3÷1.5 ·m

• for modulating burners: M = 2.0÷2.5 ·m where m is the delivery of fuel equivalent to the maximum output of the burner.

Schedule for the tabular scaling of the light oil feed pipelines

Table 14

Pipeline length (m)

Pipelines

The pipelines are dimensioned considering the flow inside the pipes under turbulent conditions, as diesel oil has low viscosity.

The pipeline pressure drops are the sum of those distributed along the pipeline and those concentrated due to connecting elements and hydraulic accessories (filters, valves, etc..).

The concentrated drops due to hydraulic accessories are reckoned using the equivalent lengths method, i.e. a concentrated loss is assimilated to a section of pipeline equivalent to the length of the related loss.

To correctly dimension the pipelines the following sizes are defined:

LEFF = effective length of the pipeline [m];

LEQUIV = sum of equivalent lengths relative to concentrated pressure drops as a result of connecting elements and hydraulic accessories [m];

LTOT = total length of the pipeline, sum of the effective and equivalent lengths [m]:

L_TOT= L_{EFF +} L_EQUIV [m] Eq 2.6.2 -2 The equivalent lengths relative to the concentrated resistances of the components must be taken from the technical specifications supplied by the manufacturer. If these values are not available, some tables exist, shown in section 5, which contain the equivalent lengths referring to the main concentrated resistances.

Any filters must be calculated using the effective pressure drop procured by their presence provided by the manufacturer. If the exact pressure drop value is not available, the filter can be assimilated to an open valve.

The calculation delivery is, obviously, equal to that of the pumping unit on the main ring.

As far as the delivery pipeline is concerned, the diameter should be chosen in relation to the maximum permitted speed equating to 1÷2 m/s using the following equation:

eq 2.6.2 -3 where:

d = internal pipeline diameter [m];

Q = delivery in terms of liquid fuel volume [m³/s] equal to m/ρ ^where ρ is the diesel oil volume mass as calculated below and m is the delivery in terms of diesel oil mass;

V = liquid fuel flow speed equal to 1.5 m/s;

A= ^QV _⇒ π .^d42 = ^{QV ⇒ d=}

√

^{4 . Qπ . V}

The chosen pipeline corresponds to the commercially available diameter immediately above that determined using the equation 2.6.2-3.

After establishing the pipeline diameter, the exact fluid speed inside the pipeline must be calculated using the equation 2.6.2-3 to establish the effective hydraulic status of the system, calculating the Reynolds Number using the following formula:

Eq 2.6.2 -4 where:

NRe = Reynolds Number;

d = internal pipeline diameter [m];

V = liquid fuel flow speed;

γ = kinematic viscosity at the transfer temperature of the liquid fuel [m²/s];

If NRe > 2,320, the flow is defined as turbulent; otherwise we have a laminar flow.

The intake pipeline, i.e. the length upstream from the pump, between the pump itself and the storage tank, must be dimensioned in relation to the maximum permissible project-related drop.

The maximum project-related pressure drop is equal to:

∆P_prog =∆P_amn - ∆h_asp - ∆P_acc [Pa]

Eq 2.6.2 -5 where:

∆P_amn= 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);

∆h_asp = intake height [Pa];

∆P_acc = head loss due to the presence of hydraulic accessories not calculated in determining the equivalent lengths on the intake pipeline (filters, etc..) [Pa]

The intake height is equal to:

∆h_{asp =} ∆h_geom . ρ. 9,81 [Pa] eq 2.6.2 -6 where:

∆h_geom = the difference in height between the fuel test point in the tank and the centre of the delivery pump [m];

ρ = diesel oil volume mass [kg/m³];

The value of ∆h_geomis 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.

N_Re= d . V γ

The liquid fuel volume mass depends on the temperature according to the following formula:

eq 2.6.2 -7

where:

ρ = liquid fuel volume mass [kg/m³];

ρ₁₅ = liquid fuel volume mass at the reference temperature of 15°C equal to 865 kg/m³; t = transfer temperature of the diesel oil equal to 2°C [°C];

β= expansion formula equal to 0.00064°C-1;

If the flow is laminar, the pipelines should be dimensioned according to the following formula:

eq 2.6.2 -8 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];

In technical practice the 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 degrees, etc…); therefore, before using the previous formula the

d= 42 . γ . L_TOT .m

∆P_prog

√

ρ= ρ₁₅ 1 + β .(t - 15)

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 -9 If the flow is turbulent, the pipelines should be dimensioned according to the following formula:

where:

d = internal pipeline diameter [m];

γ = the friction factor to be estimated in the diagram shown below in relation to the NRe and the relative texture e/D, where e represents the absolute texture in mm;

LTOT = the 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];

The table 15 shows the value of the absolute texture of certain types of pipelines:

The graph 59 shows the friction factor value f in relation to the Reynolds Number NRe and the relative texture e/D.

The diameter calculated in this manner must not, in any case, be less than 6 mm.

Once the above-mentioned diameter has been calculated, it is necessary to check that the speed is not lower than 0.15 m/s using the equation (2.6.2-3), specifically:

d=

√

⁵ 0,00084 . ^{γ . L∆PTOT prog.m2}

Material Wall status Absolute texture (mm)

Wire-drawing pipes, new (copper, brass, bronze, light alloy) Synthetic material pipes, new

technically smooth 0,001 3 ÷ 0,001 5

rolled film 0,02 ÷ 0,06

pickled 0,03 ÷ 0,04

Non-welded pipes, new

zinc-plated 0,07 ÷ 0,16

rolled film 0,04 ÷ 0,1

tarred 0,01 ÷ 0,05

Longitudinal welded pipes, new

galvanized 0,008

moderately rusted or lightly

encrusted 0,15 ÷ 0,2

Stell pipes after long employ

heavily encrusted up to 3 Absolute texture of the pipelines

Table 15

Moody's abacus Diagram 59

Pressure regulating valve Diagram 60

[m/s] eq. 2.6.2 -10

where:

d = internal pipeline diameter [m];

Q = liquid fuel delivery in volume [m³/s];

If the transfer speed is less than the limit value of 0.15 m/s, proceed as follows:

• the pipeline diameter that guarantees this minimum speed should be chosen using the formula:

eq. 2.6.2 -11

• the total maximum pipeline length (effective + equivalent) connecting the tank and the pump is determined so as not to exceed the project-related pressure drop using the following formula:

for the laminar flow

eq. 2.6.2 -12 L_TOT= d⁴ . ∆P_prog

42 . γ .m

A= Q ⇒ π . = d

V

d²

4 Q ⇒

V =

√

^{π 4 . Q . 0,15}

V= Q

A π .d² 4

= ^Q

for the turbulent flow

eq. 2.6.2 -13

The pump is situated 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 6 mm, a pipeline with an internal diameter of 6 mm should be chosen taking care to up-rate the delivery of the pump so that the fluid speed is greater than 0.15 m/s.

Pressure regulating valves

The pressure regulating valves are required to maintain the pressure in specific parts of the circuit and therefore the desired delivery. They

L_TOT= d⁵ . ∆P_prog 0,00084. γ .m

f

Reynolds number Re = ρVd / µ

Laminar flow

are installed in the main ring, normally between the intake and return pipelines from the burner pump 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 a large delivery variation, the established pressure is maintained within a certain tolerance range.

These valves are chosen on the basis of the following project data:

• delivery equal to that of the pumping unit in the related circuit;

• pressure range typically between 100,000 and 400,000 Pa (1÷4 bar).

2.6.3 Feeding of heavy oil (fuel

In document Burner Handbook (Page 56-60)