EL MODELO DE DECISIÓN OCUPACIONAL. CUATRO DIMENSIONES

WATER INDUSTRY BUILDINGSERVICES MINING

The Commerzbank headquarters in Frankfurt on the Main is the highest office building in Europe and dom- inates the skyline with its accom- plished architectural features. Impressive, too, is the number of KSB products integrated in the building’s various service systems that reliably fulfil their daily tasks

to their owner’s complete satisfac- tion: approximately 300 pumps and 2,000 valves of a wide variety of types and sizes have safeguarded safe and reliable fluid transport since 1997. Our extensive product scope, the know-how of our special- ists and KSB’s uncompromising quality management convinced the

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11

11

Dry running protection Selection criteria for PBUs

“Dry running protection” is the protection of the pumping station against inadmissible operation (such as lack of water) and the timely shutdown of the PBU to protect the supply network.

The following presents an overview of the various dry-running protection concepts.

11.1

Protection of the supply network

By stipulating minimum pressures, the General Water Supply Terms Ordinance (AVB Wasser V), the DIN 1988 and the future DIN EN 806 stand- ards endeavour to prevent consumers directly supplied via a house connection being inadmis- sibly compromised as a result of PBU operation. Possible causes for an inadmissible pressure drop at the house connection are for example:

• Insufficient supply network. The pressure of the mains water pipe drops when considerable quantities of water are extracted.

• If the service pipe’s diameter is insufficient, the supply pressure drops at the hand-over point when considerable quantities of water are con- sumed.

• Pressures drop at the hand-over point if consider- able quantities of water are pumped via the PBU (e. g. due to large pumps and/or a large mem- brane-type accumulator downstream of the PBU).

An inlet pressure monitoring device is installed on the PBU inlet side in order to monitor and observe the required minimum pressure of pmin,V= 1 bar. Depending on the unit, this monitoring equipment can either be a pressure switch or an analog pres- sure transmitter. Whenever the minimum pressure is reached the PBU should reduce the number of operating pumps.

Simultaneous stopping of all operating pumps must be avoided.

Alternatively, the possibility of an indirect connec- tion of the PBU providing sufficient protection should be examined.

11.2

Protection of pressure boosting unit High-pressure centrifugal pumps are mainly employed for PBUs and, apart from a few excep- tions, these are non-self priming pump types. This means that the inlet / suction pipe must be con- stantly filled with water.

In essence, all protection concepts aim to prevent inadmissible rises in temperature or, in the worst case, the dry running of the pumps.

A distinction should be made between direct and indirect protection methods.

Direct protection:

• Flow control on the PBU inlet side Indirect protection:

• Inlet pressure control upstream of the PBU – particularly in the case of cascade control (Hya-Eco and Hyamat K) – by making sure that the pumps can reach or exceed the cut- out pressure p_co.

• Water level control in the inlet tank

• Pump performance monitoring (electrically) • Temperature monitors on each pump (alter-

natively)

Fig. 17:

Pressure development at the consumer end of the service pipe, i. e. upstream of the PBU, during start-up and shutdown of pumps

Permissible pressure

increase

Permissible pressure drop

Period of

operation Run-down time Pump start- up phase p_min,V 0.0 bar p₂ p2 p1 p₁ ≤ 1.0 bar 1.0 bar ≤ 0.5 p_min,V

Conclusion:

Generally, PBUs must be protected against operat- ing with a lack of water.

As a whole host of connection types for PBUs and different operating conditions exists, there is no such thing as a standard solution. The protection concept chosen must always suit the individual operating conditions (see Fig. 20).

Direct connection

PBU protection concepts

Options

Flow monitoring + pressure monitoring downstream of the PBU

Water level monitoring using a float switch

Pressure monitoring using a digital pressure transmitter 욷 0.5 bar

Water level monitoring using electrodes

Pressure monitoring using an analog pressure transmitter 욷 0.5 bar

Options Options PBU PBU PBU PBU PBU Q P P P e.g. BMS

PBU not self-priming

After “lack of water” tripping manual reset required

Indirect connection

Suction head operation Suction lift operation

Fig. 20:

12

Effects of inlet pressure fluctuations Supply pressure fluctuations at the hand-over point (water meter) influence the operating behaviour of pressure boosting units.

12.1

Indirect connection, Fig. 21

The PBU takes the required quantity of water from an unpressurized inlet tank situated up- stream.

The tank filling device is usually sized such that the PBU’s nominal flow rate Q_Nis reached under normal inlet pressure (p_inl) conditions. If the inlet pressure drops to the minimum inlet pressure p_inl,minthis may lead to a significant reduction of the filling volume.

Example:

p_inl,min = 1.0 bar pinl,max = 3.1 bar

p_inl = 2 bar (angenommen) QN = 20 m3/h

Selection of float valve for p_inl= 2 bar, Fig. 22 QN = 20 m3/h 씮 11⁄2”

at p_inl,min= 1 bar 씮 Q = 14 m3_/h

This results in a filling volume reduction of 30%. It should be noted that the stored volume will be used up under minimum inlet pressure conditions and a water consumption of more than 14 m3_/h over a prolonged period of time (risking a lack-of- water condition).

The PBU’s function is not impaired as long as water is available.

12.2

Direct connection, Fig. 23 12.2.1

Inlet pressure fluctuations in the form of pressure increase

When PBUs with cascade control are used (Hya-Eco, Hyamat K, Hya-Solo D) inlet pressure fluctuations (in this case in the form of pressure increases) will directly influence the PBUs’ dis- charge pressure. Since cascade-controlled PBUs have an inherent discharge pressure fluctuation of ⌬p(co-ci)+ 0.3 bar (Fig. 24), the sum of discharge pressure fluctuation and inlet pressure fluctuation must be examined to ensure that it is acceptable for the consumers downstream. The maximum fluctuation range recommended by DIN 1988 is 2.5 bar. [31] PBU Fig. 21 Indirect connection 70 50 40 30 20 10 0 500 300 200 100 0 0 1 2 3 4 5 0 1 2 3 4 5 Q m /h3

Inlet pressure p bar Inlet pressure p bar Q m /h3 2 x 2” 2” DN 100 DN 80 1” 1 / ”1 2 14 Fig. 22

Selection diagram for filling devices

PBU

Fig. 21: Direct connection

12

Effects of inlet pressure fluctuations

Legend for [31]:

⌬pPBU,max = Maximum pressure difference

downstream of the PBU

⌬p(co-ci) = Switching pressure difference

⌬pinl,max = Maximum pressure upstream

of the PBU

If this is not the case, it is important to use an inlet side or discharge side pressure con- troller / reducer.

A negative effect of inlet pressure increase is the shifting of the minimum flow rate for continuous operation (Qcoat pco) and the minimum inlet pressure pinl,minto

considerably larger cut-out flow rates at increased inlet pressure, especially in the case of PBUs with a low number of stages.

The consequences are as follows (Fig. 24): • Low flow rates (> Q_cofor p_inl,min) lead to the

risk of excessive on/off operation (very high switching frequency of the PBU pumps). • As a result of the cut-out of the last pump in

operation at flow rates > Qcoaudible pressure surges in the pipes are to be expected.

Example:

Impact of inlet pressure increase on a real pump curve (Fig. 25), Hyamat K series.

Pump size: 0406

Qco = 1.5 m3/h at pinl,min

After an inlet pressure increase of 0.5 bar pinl= pinl,min+ 0.5 bar

the cut-out flow rate is Qco = 3 m3/h.

Cascade-controlled PBUs have fixed pressure- dependent switching points for pump cut-in and cut-out.

In the case of direct PBU connection, the respec- tive inlet pressure is added to the discharge head of the pump(s).

The factory-set value for the cut-out pressure p_cois normally 0.3 bar lower than the maximum

discharge pressure of the pump (at Q = 0 m3_/h). Point 1 on the pump curve (Fig. 25) characterizes the cut-out flow rate of the pump for the design inlet pressure pinl,min(not considering possible after-run time).

As a pump is always stopped whenever the cut-out pressure p_cois exceeded, the conclusion in the case of increased inlet pressure is that the pump stops with a lower discharge pressure.

Point 2 demonstrates the increased cut-out flow rate at the lower pump discharge pressure. The characteristic curves used as an example clearly show that especially with flat pump curves (fewer stages) one can expect an excessive flow rate increase at the cut-out point.

Fig. 24:

The operating characteristics of a pressure boosting unit with cascade control, i. e. without speed control

Impact of inlet pressure increase of 0.3 bar

Discharge pressure

Pressure fluctuation inherent to PBU

p (co - ci) p (co - ci) + p inl,min + 0.3 bar p E p A H 0 Q co Q co Q D + 0.3 bar Example:

Pressure development of a system with 3 pumps with increasing flow rate (consumption) with decreasing flow rate (consumption)

12

Inlet pressure fluctuations in the form of pressure decrease

12.2.2

Inlet pressure fluctuations in the form of pressure decrease

Inlet pressure fluctuations (in this case in the form of pressure decreases) which fall below the min- imum inlet pressure are particularly dangerous for PBUs.

Cascade-controlled PBUs have a nominal cut-out pressure margin of 0.3 bar. This means that the maximum pump discharge head is 0.3 bar above the cut-out pressure pco. As soon as the minimum inlet pressure drops by more than 0.3 bar, the PBU

pumps in operation can no longer reach the cut- out pressure!

0 Pumps per system

1 Point 1 Point 2 3.0 2 4 6 8 10 12 2 3 4 5 6 / / 2 3 4 5 6 3 6 9 12 15 18 4 8 12 16 20 24 5 10 15 20 25 30 6 12 18 24 30 36 7 14 21 28 35 42 8 16 24 32 40 48 9 18 27 36 45 54 14.4 p bar A H m Q m /h3 160 150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0 0415 0413 0411 0409 0408 0407 0406 0405 0404 0403 0402 12.4 10.5 9.5 8.5 6.6 5.6 4.6 3.6 2.6 1.7 0 0.5 bar 7.5 0410 1.5 without stand- by pump with stand- by pump Characteristic curve valvues and tolerarances to ISO 2548, Annex B

Specifically selected PBU for p inl,min

Fig. 25:

13

Causes of pressure surges 13.1

Pressure surges caused by valves

All kinds of valves can, when they close quickly, lead to pressure surges.

The theoretic basis for calculating this phenom- enon is described in the formula by Joukowsky:

[32]

Legend for [32]

⌬H = Pressure increase

a = Propagation of a pressure wave at approx. 1000 - 1200 m/s

g = Acceleration due to gravity, approx. 10 m/s2

⌬v = Velocity difference

The full impact of the pressure surge can be math- ematically expressed by the following formula:

[33]

Legend for [33]

T_C = Valve closing time

TR = Reflection time in the piping

l = Length of piping up to the location of disturbance

This means when the pressure wave returns and the valve is closed, the pressure surge according to Joukowsky fully develops.

Remedy:

T_Cmust be markedly longer than T_R.

e.g.: TC 욷 2 · TR

13.2

Pressure surges as a result of inlet pressure fluctuations in the case of PBUs with cascade control

If the inlet pressure increases, the pump curves shift upwards and the base load pump is stopped at a higher volume flow (Fig. 27). This results in an increased flow velocity change as the pump stops, thus generating pressure surges!

Remedy:

A pressure reducer/controller has to be installed upstream of the pressure boosting unit to prevent detrimental effects.

Impact of inlet pressure increase by 0.3 bar

Discharge pressure

Example:

Pressure development of a system with 3 pumps with increasing flow rate (consumption)

with decreasing flow rate (consumption)

p (co -ci)

p E p A H 0

Q co, normal Q co, inlet pressure increase by 0.3 bar Q D

Fig. 27:

Operating characteristics of a pressure boosting unit with cascade control, i. e. without speed

l Valve pressure wave Location of disturbance, e.g. T-piece Fig. 26: a ⌬⌯ = — · ⌬v [m] g 2 · l TC 울 TR = ———_a [s]

14

Standards, directives and statutory regulations

14

Standards, directives and statutory regulations

DIN 1988 (December 1988 edition)

Codes of practice for drinking water installations (TRWI); general; - DVGW1_{code of practice}

Part 1, General

includes among others:

Competences, technical terms, graphic symbols and designations

Part 2, Planning and operation; component parts, apparatus and materials

includes among others:

Piping units, valves, apparatus, drinking water heating units, drinking water tanks, drinking water treat- ment equipment, measuring and metering instruments.

Part 2, Sheet 1

Compilation of standards and other technical specifications regarding materials, components, apparatus.

Part 3, Calculating the pipe diameters

includes among others:

Calculation flow rate and flow pressure, application of conversion curve(s) of total flow to peak flow, selection of piping diameter, circulation pipes and circulation pumps, calculation examples; Annex A: forms for calculation

Part 3, Sheet 1, Calculation examples

Part 4, Protection of drinking water, drinking water quality control

includes among others:

Causes for impairment and endangering of drinking water quality, equipment preventing back flow, protection of drinking water in water heating units

Part 5, Boosting and pressure reduction

includes among others:

Pressure boosting units and pressure reducers

Part 6, Fire-fighting and fire protection installations

includes among others:

Terms, configuration and requirements, commissioning

Part 7, Prevention of corrosion damage and calcareous deposit formation

includes among others:

Part 8, Operation of plants

includes among others:

Commissioning, operation, damage and faults, maintenance information Annex A Information on maintenance measures

Annex B Inspection and maintenance schedule

Annex C Sample forms for commissioning and instruction reports

DIN 2000 (November 1973 edition)

Centralized drinking water supply; principles for drinking water requirements, planning, construction and operation of units

DIN 2001 (February 1983 edition)

Individual drinking water supply; principles for drinking water requirements, planning, construction and operation of units; DVGW code of practice

DIN 4109, Part 5 (October 1989 edition)

Sound insulation in building construction

EN 1717 (February 1995 edition)

Protection against pollution of potable water installations and general requirements of devices to prevent pollution by backflow

DIN 4807, Part 5 (March 1997 edition)

Direct flow membrane pressure vessels in drinking water installations

DIN 14200 (June 1979 edition)

Nozzle discharge

IfSG – Protection Against Infection Act (01.2001)

Act regulating the prevention of and protection against communicable diseases in humans

LMBG – Foodstuffs and Commodities Act (09.1997)

Official food control law for food, tobacco, cosmetic products and other commodities to protect health and to safeguard consumers’ rights

TrinkwV 2001 – Drinking Water Ordinance

Regulation for the quality of water for human use

VDI Directives

Directives issued by the Association of German Engineers

Standards, directives and statutory regulations

14

Standards, directives and statutory regulations

Group Group I Group III Group IV

classification

울 PN 25

p · V 울 200 p · V > 200; p · V > 1000; > 1 bar p · V 울 1000

Operating pressure > 1 bar Operating pressure > 1 bar

Single tests Pressure vessel code (DruckbehV) § 9, Testing prior to commissioning

In the case of a pressure vessel which has been subjected to an ac- ceptance test at another location – excluding the installation test – and not applicable provided with an acceptance certificate accordingly, it is sufficient that

the installation at the place of operation is checked by an expert and that this is confirmed in writing, e. g. through a commissioning/acceptance report

Pressure vessel code (DruckbehV) § 10 Regular testing

(1) In accordance with sections 4 to 9, a group IV pressure vessel must be subjected to regular testing carried out by an expert (e. g. TÜV) at defined intervals. Regular testing not applicable not applicable

(4) Groups IV and VII pressure vessels must be subjected to internal tests every five years; pressure tests must be carried out every ten years and external tests must be performed every two years.

Allocation of Accumulator PN 10 ; 80 l PN 10; 180 l, 300 l, 400 l, 800 l, 1000 l membrane type (PN 16; 12 l) PN 16; 80 l, 180 l, 300 l, 400 l, 800 l, 1000 l pressure vessel

(accumulator) sizes

Regulation governing plant safety (BetrSichV, 27.09.2002 edition)

Excerpt (special, manufacturer-related aspects have to be considered)

Table 2:

15

Inspection, maintenance and repair to DIN 1988, Part 8

Inspection, maintenance and repair: According the manufacturers’ operating instructions

Carried out by: Installation contractor

Interval: Annually unless otherwise specified by the manufacturer

Inspection, maintenance and repair

16

Nomenclature

16

Nomenclature

Description Symbols Unit

Propagation velocity of a pressure wave a m/s

Simultaneity factor f -

Acceleration due to gravity (9.81 m/s2_{) g m/s}2

Pump discharge head H m

Max. pump discharge head at zero flow rate (Q = 0) H₀ m

Static head loss H_geo m

Floor height H_fl m

Head loss H_L m

Head increase ∆H m

Static pressure loss for floor X ∆H_{geo (floor X)} m

Number of pumps (incl. stand-by pump) n -

Number of floors which have to be protected against N_flpr - inadmissible pressures by means of pressure reducers

Number of floors which can be supplied without PBU N_{without PBU} -

Pressure p bar

Maximum pump pressure at zero flow rate (Q = 0) p₀ bar

Cut-out pressure, pressure at which one pump of a p_co bar

pressure-controlled PBU cuts out

Cut-in pressure, pressure at which one pump of a p_ci bar

pressure-controlled PBU cuts in Setpoint (of a speed controlled PBU)

Flow pressure at consumer p_flow bar

Minimum flow pressure at consumer p_min,flow bar

Minimum flow pressure at consumer in floor N p_min,flow(N) bar Minimum supply pressure (at hand-over point between

local provider and consumer) p_min,V bar

Maximum pressure p_max bar

Maximum flow pressure at consumer p_max,flow bar

Maximum pressure per floor p_max,fl bar

Maximum supply pressure (at hand-over point between

local provider and consumer) p_max,V bar

Required pressure downstream of the PBU p_discharge bar

Available pressure upstream of the PBU p_inl bar

Minimum available pressure upstream of the PBU p_inl,min bar

Maximum pressure upstream of the PBU p_inl,max bar

Description Symbols Unit

Pressure drop during pump start-up ∆p1 bar

Pressure increase during pump shutdown ∆p₂ bar

Apparatus pressure loss ∆p_ap bar

Difference between cut-in/-out pressures ∆p_(co-ci) bar

Maximum pressure difference downstream of the PBU ∆p_PBU,max bar

Dynamic pressure difference ∆p_dyn bar

Dynamic pressure loss per floor ∆p_dyn,fl bar

Static pressure loss ∆p_geo bar

Static pressure loss per floor ∆p_geo,fl bar

Static pressure loss for floor X ∆p_{geo (floorX)} bar

Pressure loss per floor ∆p_fl bar

Total pressure loss per floor ∆p_fl,tot bar

Pressure fluctuation upstream of the PBU ∆p_inl bar

Water meter pressure loss ∆p_wm bar

Permissible pressure loss ∆p_perm. bar

Mean pressure drop in consumer supply pipes ∆p/l mbar/m

Cut-out flow rate of the last pump in operation (without after-run period) Q_co m3_/h PBU design flow rate = nominal flow rate of a PBU (V·_max) Q_D m3_/h

Maximum volume flow of a PBU incl. stand-by pump Q_DS m3_/h

Fire-fighting water volume flow Q_Ffw m3_/h

Critical minimum flow rate of pressure-dependent cascade control Q_crit m3_/h

Minimum flow rate Q_min m3_/h

Maximum permissible volume flow of filling device Q_max m3_/h Maximum permissible volume flow from water supply network Q_max,netw m3_/h

Nominal flow rate of water meters Q_n m3_/h

Nominal flow rate of PBU Q_N m3_/h

Filling volume flow Q_fill m3_/h

Water supply network volume flow Q_netw m3_/h

Low or permanent consumption dependent on usage Q_{consumer, small} m3_/h Nomenclature

16

Nomenclature

Description Symbols Unit

Volume flow difference ∆Q m3_/h

Volume flow change of a highly dynamic consumer ∆Q_max,dyn m3_/h

Change of nominal flow rate per pump ∆Q_PU m3_/h

Switching frequency S -

Period of time required to fill a hydrant pipe up to the wall-mounted t s fire hydrant which is most unfavourably located

Inlet tank buffering period t h

Time difference ∆t s

Closing time of valve T_C s

Reflection time in the pipeline T_R s

Flow velocity v m/s

Flow velocity difference ∆v m/s

Usable volume of the atmospheric inlet tank V_IT m3

Accumulator volume V_accum. m3

Gross accumulator volume V_gross m3

Total accumulator volume V_tot m3

Contents of network piping V_pipe m3

Calculation flow rate of a tap/fitting V_cal m3

Total accumulator volume V_tot m3

Volume difference ∆V m3

Nominal flow rate of water meters V·_n m3_/h

Minimum flow rate V·_min m3_/h

Nominal flow rate of a PBU = design flow rate of a PBU (Q_D) V·_max m3_/h

Maximum volume flow of a PBU pump V·_max,p m3_/h

Calculation flow rate of a wall-mounted fire hydrant V·_cal l/s

Peak flow rate of a PBU V·_peak l/s

Sum of all calculation flow rates of all taps/fittings to be supplied ΣV·_cal l/s

Pipe friction and other losses Σ(R · I+Z) bar

Pipe friction and other losses from supply pipe up to PBU Σ(R · I+Z)_inl bar Pipe friction and other losses downstream of PBU Σ(R · I+Z)_discharge bar

Mean pressure drop in a pipe R bar/m

Individual friction losses Z bar

Pipe length I m

Sum of pipe length from PBU to the hydraulically least favourable extraction point ΣI_discharge m

Accumulator efficiency e -

In document EMPRESA, ECONOMÍA Y SOCIEDAD. HOMENAJE A VICENTE SALAS FUMÁS VOLUMEN II (página 174-181)