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2.1.2. La oferta de segundo ciclo: la oferta de Masters desde la Geografía
manufacturing and installation debris from hydraulic systems by flushing
Flushing
procedures
Fig. 1. Simplified sketch of experiment that Reynolds used to study and define the three regimes of fluid flow.
Turbulent flow Transition flow (or critical zone)
Laminar flow
Dye Dye injector
Valve H2O
1T. B. Hardison, "Fluid Mechanics for Technicians," Reston Publish-ing, 1977.
2Vickers Inc., "Vickers Industrial Hydraulic Manual," 1989, page C-16.
draulic fluid is influenced by tempera-ture and pressure. That is, the hotter the oil, the higher the NRfor the same fluid velocity and pressure. The higher the pressure, the lower the NRfor the same fluid velocity and temperature. Thus, specifying that NRshould be 3000 is not a stringent requirement, but is well within the normal operating fluid ve-locities of a system. By definition, tur-bulent flow has been created because the fluid streamlines are no longer par-allel, but sufficient fluid motion to clean the inside walls of the conductors has not been generated.
Even at the recommended maximum fluid velocities and NRs for hydraulic-system working conductors, fluid flow still is not turbulent enough to greatly affect contamination on conductor walls. Boundary-layer fluid at the inte-rior surfaces of the fluid conductor re-mains undisturbed.
Second example
The NRfor flow at normal system ve-locities next can be calculated using the same conductor size and kinematic
vis-cosity as in the first example, but with the velocity increased to 20 ft/sec. This higher velocity results in a Reynolds Number of 11,671, which corresponds to a flow rate of 39.8 gpm.
As NRincreases, flow conditions go from laminar, through the critical zone, to turbulent. It has been proven empiri-cally that once NRexceeds 3000, resis-tance to fluid flow is a combination of the effects of turbulence and of viscous drag at the conductor wall. (This region of viscous drag at the conductor wall is known as the viscous sub-layer.) There is a transition zone within the turbulent flow range where flow resistance goes from being governed by turbulence ef-fects to being governed by the rough-ness of the inside wall of the conductor.
This is shown clearly when in-specting the Moody diagram,3Figure 2, which graphically demonstrates the relationship between Reynolds
Number NR, friction factor f, and e, the roughness of the conductor’s in-s i d e in-s u r f a c e . R e in-s i in-s t a n c e t o f l o w through a fluid conductor — as repre-sented by friction factor f — is only affected by the surface roughness of the fluid conductor4when NRis above 4000. This means that the majority of the resistance to flow is created by turbulence effects. Only when NRis high enough so that surface projec-tions of the conductor walls extend beyond the viscous sublayer does the surface come in contact with the tur-bulent flow and affect the pressure drop in the conductor.
Surface roughness
For drawn tubing, average surface roughness e is 0.000005 ft. If the con-ductor is the same 1-in. tubing with 0.049-in. wall thickness, ratio e/D will be 0.000067. The Moody diagram indi-cates that for this conductor, NRmust be at least 25,000 before the inside surface exposes its resistance to fluid flow.
Therefore, to ensure the inside wall of the conductor will be cleaned, NRmust
1998/1999 Fluid Power Handbook & Directory
A/85 F L U S H I N G P R O C E D U R E S
0.05 0.04 0.03 0.02 0.015 0.01 0.008
0.006 0.004
0.002 0.001 0.0008 0.0006 0.0004
0.0001
0.0001 0.00005
0.00001
0.0003 e/D
0.008 0.009 0.01 0.015 0.1 0.09 0.08 0.07 0.06 0.05
0.04
0.03
0.025
0.02
0.0002 f
Critical
zone Transition
zone
NR
103 104 105 106 107 108
Laminar
flow Rough
Smooth
Fig. 2. Modified Moody diagram represents relationship of friction factor f, Reynolds Number NR, and conductor surface roughness e.
3Vennard & Street, "Elementary Fluid Mechanics," John Wiley & Sons, Inc., 1982, page 381.
4Also from reference 3.
be greater than 25,000. For flow to be fully in the rough zone of turbulent flow, NRmust be greater than 3.253107. Us-ing 1.28831024ft2/sec, the same fluid kinematic viscosity as in the first exam-ple, a NRof 25,000 corresponds to a fluid velocity of 42.8 ft/sec, or a flow rate of 85 gpm — still easily attainable with conventional hydraulic pumps.
Real-world systems
It can be argued that if the walls of a conductor are not greatly affected by normal system fluid velocities, contaminants lodged there will have little chance of entering the fluid stream. This may be partially true but the argument applies only to smooth, straight conductors at steady flows and pressures. It is not representative of normal installations that combine straight runs, bends, and numerous fittings where flow patterns are only predictable empirically, and where pressure fluctuations and spikes are commonplace.
Depending on the severity of ser-vice that the system will experience, pressure spikes will dislodge contam-inants held in the walls of the conduc-tors and between fitting interfaces.
Remember that in critical systems, 3-to 25-µ m particles can impact system performance significantly. The only way to guarantee that conductor con-tamination (that can be released at any time during operation) does not affect system performance is to pro-tect each component with a filter, an option so costly that it would not be u s e d i n m o s t s y s t e m s . A l t h o u g h flushing hydraulic system conductors at the normal system operating-fluid velocities can provide fluid velocities higher than flushing at a NRof 3000, the inside wall of the conductors still will not be cleaned.
High-velocity/high-pressure flushing Flows that produce NRs .25,000 are needed to ensure that conductor walls are exposed to turbulent flow.
Because system conductors may con-sist of pipe, tube, and/or hose and as-sociated fittings, the specification of a contractual number for NRis
diffi-cult and still does not guarantee that conductors will be cleaned. The best one can do is establish conditions that will maximize NR. These conditions are: the highest possible velocity at the lowest possible fluid viscosity.
Limiting factors are the conductor’s pressure rating and the fluid’s maxi-mum operating temperature.
When flushing a system, the valv-ing and actuators must be “jumpered”
for safety reasons so the only resis-tance to fluid flow is the pressure drop in the conductors and fittings.
When flow becomes turbulent, the pressure drop is proportional to the square of the velocity. Extrapolating this relationship to its maximum, the h i g h e s t p o s s i b l e v e l o c i t y o c c u r s when the pressure drop in the con-ductor generated by fluid flow is equal to the maximum test pressure of the conductor. Flushing a system at these high flows and pressures has the added advantage of expanding and contracting the conductors and fittings as the pressure fluctuates while inducing highly turbulent flow.
This optimizes the flushing action.
By equating the pressure drop in a conductor to the maximum pressure rating of that conductor, the maxi-mum fluid velocity possible, along with the corresponding NR,can be calculated. The temperature of the fluid directly affects its viscosity and is the other variable that can control NR. Flushing pressure also affects viscosity, but this is hard to quantify because pressure in the pipe being flushed will vary from maximum at the pumping source to atmospheric at the conductor outlet.
T h e e q u a t i o n u s e d t o c a l c u l a t e head loss in the turbulent zone is:
hl5 fLV2/2D,
where: hlstands for head loss, f is the friction factor found in the Moody diagram,
L is the conductor length in ft, V is the fluid velocity, and D is the conductor’s ID in in.
This equation will calculate the maxi-mum velocities and NRs that can be achieved for a given maximum pressure.
Determining f for pipe flow re-quires iterative calculations using the Moody diagram. Given the pressure rating, ID, length, and relative rough-ness of the conductor, assume an f and then calculate the fluid velocity. next calculate NRand determine a new f from the Moody diagram. Repeat the calculation until f converges.
The table above contains velocities and NRs that have been calculated for 200 feet of Schedule-80 pipe using the maximum test pressure for the pipe and a surface roughness of 0.00015 ft for wrought iron pipe. These calculations did not take into account the pressure drop produced by the various fittings normally used, so the values for the at-tainable fluid velocities and NRs are op-timistically high. Also, special fluids with lower viscosities or flushing at higher temperatures to reduce the fluid viscosity can increase NR.
The values determined for maximum flushing velocity and flow rate indicate that some of these conditions, mainly for lines smaller than 3/4in., can be satisfied using conventional high-pressure pumps of appropriate size, although it may be difficult to induce the pressure fluctua-tions needed to dislodge contaminants.
For systems with larger conductors, spe-cial methods must be used to achieve the necessary pressures, fluid velocities, and NRs to properly flush the lines.
F L U S H I N G P R O C E D U R E S
Pipe size Test pressure Relative Maximum Flushing
- in. D - psi roughness - e/D velocity - fps flow rate - gpm NR
1/2 0.546 3500 0.0033 65 48 23,000
3/4 0.742 2900 0.0024 74 100 35,000
1 0.957 2700 0.0019 84 188 52,000
11/2 1.500 2100 0.0012 96 528 93,131
2 1.939 1830 0.0009 117 1077 146,722
Calculated flushing velocities and NRs for 200 ft of Schedule 80 pipe
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H
ydraulic systems can use either of two methods of load control:the energy-loss method, in which flow to the actuator is limited by valving, or the volume-control method, in which the stroke of a variable-dis-placement pump controls the rate at which pressure fluid is sent to the actu-ator. While the latter method is inher-ently more efficient, a system using that design principle is more costly and reacts less rapidly.
Most industrial hydraulic systems are designed using the energy loss method. Such systems cost less to build and are more responsive because sys-tem energy is immediately available.
But because of the inherent poor effi-ciency of these systems, energy losses in the form of heat can sometimes ap-proach the prime mover’s nameplate horsepower. As an example of this heat build-up, even well-designed electro-hydraulic servovalve or proportional valve systems may convert 60 to 80%
of input horsepower to heat. Well-de-signed non-servo systems may produce heat losses of 20 to 30%.
Some hydraulic system heat is desir-able to bring fluid up to operating tem-perature. Cold hydraulic oil has a higher viscosity than warm oil. So maintaining an operating temperature of 1008 F would cause sluggish opera-tion and excessive pressure drop in a system designed to operate at 1408 F.
When a system begins operation on a cold winter morning, for example, the oil should be allowed to warm until it reaches a temperature where heat is generated at the same rate as system heat radiating into the atmosphere or other cooling medium.
If heat generation exceeds the radia-tion rate, the excess heat can cook the oil, start oil decomposition, form var-nish on system component surfaces, and begin deteriorating system seals.
Excess heat sooner or later spells
trou-The same is true for the overall heat transfer coefficient, U.
Heat dissipates from a fluid system through natural and forced convection.
Natural convection occurs as heat moves from system components into the surrounding atmosphere because of the temperature gradient. In smaller hy-draulic systems, temperatures gener-ally are lower than in larger systems, and heat transfer from the oil to tubing and other component surfaces often provides sufficient cooling.
But if this natural convection cannot remove enough generated heat, a heat exchanger must be installed to control system temperature. The heat ex-changer uses forced convection to re-move heat. Another mode of heat trans-mission, radiation, may often occur, but its effect is small and usually can be ignored. Generally, a heat exchanger is necessary for a hydraulic system if:
● a specific oil temperature limitation is necessary to stabilize oil viscosity
● cycle dwell time is a major portion of the total duty cycle, especially in systems with fixed-displacement pumps, and
● problems with hot oil or shortened oil or seal life have been encountered with similar systems.
Heat-transfer mechanisms
Considering shell-and-tube heat ex-changers, U is composed of several heat-transfer mechanisms. The first is the convective heat transfer from the hot fluid in the shell to the tube wall.
This can be called the hot fluid thermal resistance, which depends primarily on physical and thermal fluid proper-ties. Geometric tube bundle patterns (square or triangular centerline spac-ing when viewed from the tube ends) help the oil flow turbulently over the tubes. The baffles inside the shell in-crease the distance the oil must travel through the exchanger. This increases ble for any hydraulic system. Too much
heat breaks down oil, damages seals and bearings, and increases wear on pumps and other components. The so-lution to these problems is the inclusion of a properly sized heat exchanger as a component of the system.
Thermodynamics
Heat is a form of energy that trans-fers from one region to another be-cause of a temperature difference (gra-dient) between the regions. Just as liquids naturally flow downhill, a tem-perature gradient is a condition in which heat energy naturally flows from the hotter region to the cooler re-gion. The rate of heat transfer is an im-portant consideration in determining how quickly a heat exchanger can re-move heat from a system. A small heat exchanger with a high heat transfer rate can remove as much heat from a system as a larger heat exchanger with a lower heat transfer rate.
The defining equation for any heat exchanger is
q5UADT,
where q represents the heat load transferred in BTU/hr,
U stands for the overall heat transfer coefficient in BTU/hr-ft2-8 F,
A denotes the heat transfer surface in ft2, and
DT signifies the fluid temperature difference in 8 F.
These three factors take varying forms depending on the heat exchanger and the application involved.
Inspecting the equation shows that as each term on the right increases, more heat will transfer. If a larger sur-face area comes in contact with the heated fluid, more heat is removed. A temperature gradient between the heated oil and the cooler region toward which the heat flows also increases q.