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Saturday, February 21, 2009

Hazardous Environment - Motors

Atmospheres can be classified as:

* Non-explosive atmosphere; the atmosphere does not contain explosive elements and all types of standard products can be used.
* Explosive atmosphere; the atmosphere does contain potentially explosive elements, either gas or dust.
Explosive atmosphere is referred to as 'Hazardous area' in IEC countries and 'HAZLOC' in North America

Equipment for explosive atmosphere is designed, installed, operated and maintained according to International standards and local regulations dedicated to this area.

Hazardous Area or Location is defined by 4 elements:
* Class
* Zone
* Group
* Temperature Class


Explosive atmosphere

The relevant parameters below characterize the potentially explosive atmosphere:

* Frequency with which potentially explosive atmosphere may exist
* Capability of gas or dust atmosphere to create an explosion

Explosive gas and dust is classified according to its likelihood to be ignited, according to its characteristics:

* Minimum ignition energy
* Minimum ignition temperature
* Auto-ignition temperature
* Layer ignition temperature"

Friday, February 20, 2009

Converting Pump Head to Pressure

Converting head in feet to pressure in psi
Pump curves in feet of head can be converted to pressure - psi - by the expression:

p = 0.434 h SG (1)

where
p = pressure (psi)
h = head (ft)
SG = specific gravity

Converting head in meter to pressure in bar
Pump curves in meter of head can be converted to pressure - bar - by the expression:

p = 0.0981 h SG (2)

where
h = head (m)
p = pressure (bar)

Converting head in meter to pressure in kg/cm2
Pump curves in meter of head can be converted to pressure - kg/cm2 - by the expression:

p = 0.1 h SG (2b)

where
h = head (m)
p = pressure (kg/cm2)

Converting Pressure to Head

Since pressure gauges often are calibrated in pressure - psi or bar, it may be necessary with a conversion to head - feet or meter, commonly used in pump curves.

Converting pressure in psi to head in feet

h = p 2.31 / SG (3)

where
h = head (ft)
p = pressure (psi)

Converting pressure in bar to head in meter

h = p 10.197 / SG (4)

where
h = head (m)
p = pressure (bar)

Converting pressure in kg/cm2 to head in meter

h = p 10 / SG (4b)

where
h = head (m)
p = pressure (kg/cm2)

Motor Enclosures

The selection of a motor enclosure depends upon the ambient and surrounding conditions. The two general classifications of motor enclosures are open and totally enclosed. An open motor has ventilating openings which permit passage of external air over and around the motor windings. A totally enclosed motor is constructed to prevent the free exchange of air between the inside and outside of the frame, but not sufficiently enclosed to be termed air-tight.

These two categories are further broken down by enclosure design, type of insulation, and/or cooling method. The most common of these types are listed below.

Open Dripproof - An open motor in which all ventilating open-ings are so constructed that drops of liquid or solid particles falling on the motor at any angle from 0 to 15 degrees from vertical cannot enter the machine. This is the most common type and is designed for use in nonhazardous, relatively clean, industrial areas.

Encapsulated - A dripproof motor with the stator windings com-pletely surrounded by a protective coating. An encapsulated motor offers more resistance to moisture and/or corrosive en- vironments than an ODP motor.

Totally Enclosed, Fan-Cooled - A enclosed motor equipped for external cooling by means of a fan integral with the motor, but external to the enclosed parts. TEFO motors are designed for use in extremely wet, dirty, or dusty areas.

Explosion-Proof, Dust-Ignition-Proof - An enclosed motor whose enclosure is designed to withstand an explosion of a specified dust. gas, or vapor which may occur within the motor and to prevent the ignition of this dust, gas, or vapor surrounding the motor. A motor manufacturer should be consulted regarding the various classes and groups of explosion-proof motors avail-able and the application of each.

Motor insulation is classified according to the total allowable temperature. This is made up of a maximum ambient temperature plus a maximum temperature rise plus allowances for hot spots and service factors. Class B insulation is the standard and allows for a total temperature of 130 degrees C. The maximum ambient is 40 degrees C, and the temperature rise is 70 degrees C, for ODP motors and 75 degrees C for TEFC motors."

Net Positive Suction Head (NPSH) and Cavitation

The Hydraulic Institute defines NPSH as the total suction head in feet absolute, determined at the suction nozzle and corrected to datum, less the vapor pressure of the liquid in feet absolute. Simply stated, it is an analysis of energy conditions on the suction side of a pump to determine if the liquid will vaporize at the lowest pressure point in the pump.

The pressure which a liquid exerts on its surroundings is dependent upon its temperature. This pressure, called vapor pressure, is a unique characteristic of every fluid and increased with increasing temperature. When the vapor pressure within the fluid reaches the pressure of the surrounding medium, the fluid begins to vaporize or boil. The temperature at which this vaporization occurs will decrease as the pressure of the surrounding medium decreases.

A liquid increases greatly in volume when it vaporizes. One cubic foot of water at room temperature becomes 1700 cu. ft. of vapor at the same temperature.

It is obvious from the above that if we are to pump a fluid effectively, we must keep it in liquid form. NPSH is simply a measure of the amount of suction head present to prevent this vaporization at the lowest pressure point in the pump.

NPSH Required is a function of the pump design. As the liquid passes from the pump suction to the eye of the impeller, the velocity increases and the pressure decreases. There are also pressure losses due to shock and turbulence as the liquid strikes the impeller. The centrifugal force of the impeller vanes further increases the velocity and decreases the pressure of the liquid. The NPSH Required is the positive head in feet absolute required at the pump suction to overcome these pressure drops in the pump and maintain the majority of the liquid above its vapor pressure. The NPSH Required varies with speed and capacity within any particular pump. Pump manufacturer's curves normally provide this information."

Thursday, February 19, 2009

Specific Speed and Pump Type

Specific speed (Ns) is a non-dimensional design index used to classify pump impellers as to their type and proportions. It is defined as the speed in revolutions per minute at which a geometrically similar impeller would operate if it were of such a size as to deliver one gallon per minute against one foot head.

The understanding of this definition is of design engineering significance only, however, and specific speed should be thought of only as an index used to predict certain pump characteristics. The following formula is used to determine specific speed:



Where
N = Pump speed in RPM
Q = Capacity in gpm at the best efficiency point
H = Total head per stage at the best efficiency point

The specific speed determines the general shape or class of the impeller as depicted in Fig. 3. As the specific speed increases, the ratio of the impeller outlet diameter, D2, to the inlet or eye diameter, Di, decreases. This ratio becomes 1.0 for a true axial flow impeller.

Radial flow impellers develop head principally through centrifugal force. Pumps of higher specific speeds develop head partly by centrifugal force and partly by axial force. A higher specific speed indicates a pump design with head generation more by axial forces and less by centrifugal forces. An axial flow or propeller pump with a specific speed of 10,000 or greater generates it's head exclusively through axial forces.

Radial impellers are generally low flow high head designs whereas axial flow impellers are high flow low head designs.

Values of Specific Speed, Ns

PAPER STOCK PERCENT CONSISTENCY

The consistency of a pulp and water suspension is the percent by weight of pulp in the mixture. Oven Dry (O.D.) consistency is the amount of pulp left in a sample after drying in an oven at 212?F. Air Dry (A.D.) consistency is an arbitrary convention used by paper-makers, and is the amount of pulp left in a sample after drying in atmosphere. Air Dry stock contains 10% more moisture than Bone Dry stock, i.e. 6% O.D. is 6.67% A.D."

Tuesday, February 17, 2009

API 610 Power rating for motor drives


Pump selection and quality considerations

The following conditions should (explicitly or implicitly) be known in view of correctly selecting a pump:

  1.  task of the pump in the system.
  2. The system pressure and temperature.
  3. Data for rated performance: QR, HR,tot. Often rated performance equals the guaranteed point Qg, Hg. The rated and/or guaranteed performance may beidentical to the BEP (but this is not necessarily so).
  4. The NPSHA of the plant at rated, guaranteed or BEP conditions and, as necessary,at other operation conditions.
  5. Performance data for other specific operation points (if necessary).
  6. The maximum and minimum flow rates in the domain of continuous operation.
  7. The maximum and minimum flow rates during short-term operation or in transient conditions, e.g. during a switch-over of parallel working pumps, at load rejection or other.
  8. . For pumps operating in parallel the maximum flow rate (run-out) is determined by the operation of a single pump. At run-out the available NPSHA must be sufficient to prevent excessive cavitation.
  9. When pumps are installed in series, their interaction has to be analyzed with regard to control and upset conditions such as one pump falling out of service.
  10. The type and the chemical composition of the medium to be pumped, in particular corrosive substances.
  11. The physical properties of the pumpage if it is any other than water or a common, clearly defined medium. In this case the vapor pressure must be correctly specified in order to ensure that the effects and risks of cavitation can be assessed.
  12. Viscosities appreciably above that of cold water need corrections for Q, H, P,η and NPSH .
  13. Possible inclusions of free gas or dissolved gases that might separate from the liquid in the suction pipe.  The available NPSHA must be selected so that the volume fraction of free gas at the impeller inlet is below typically 2 to 4% at low suction pressures.
  14. Possible inclusions of solids (abrasion).
  15. The type of driver (electric motor, turbine, combustion engine).
  16. Fixed or variable speed? Speed range, if applicable.
  17. Is a gear box necessary?
  18. What kind of control is intended?
  19. How much standby capacity is required (e.g. 2x100% or 3x50% pumps)?
  20. Operation mode: Continuous or short-term operation? Cyclic operation with frequent start-ups and shut-downs?
  21. Installation conditions: Horizontal or vertical arrangement?
  22. Approach flow or suction conditions: Open or closed circuit? Open pit?
  23. Fluid level variations in the suction and discharge reservoirs or pressure variations on the suction and discharge side of the pumping system.
  24. The system characteristic or at least its static part Hstat resulting from the geodetichead differences and/or the pressure differences between the suction tank and the discharge vessel.
  25. Are there any special requirements concerning the head-capacity characteristic (steepness, head rise, shut-off pressure)?
  26. The maximum admissible shut-off pressure with the allowed tolerance, if applicable.
  27. For correctly sizing the driver, the maximum power consumption must be determined; with a small specific speed it occurs at about the maximum flow rate, with a medium nq near the BEP, and with very high specific speeds at shut-off.
  28. Are there any special requirements regarding vibrations or noise? Have limits been specified for the sound level?.
  29. What tolerances are permitted for manufacturing and measurements? Which standard is to be applied for the acceptance test?
  30. The guarantee and acceptance conditions, including possible penalties on efficiency or power consumption
  31. The operation period per year and the energy costs (e.g. $/kWh) or an assessment of the capitalized energy costs (e.g. $/kW). Minimization of the energy costs per year according to the intended operation scenarios.
  32. Safety considerations, explosion protection, zero-leakage to environment, ecological aspects.


Monday, February 16, 2009

Electric Motor Speed


The speed of an AC electric motor is determined by the frequency of the supply and the number of poles in the motor stator according to the following relationship:

where
n = speed (r/min)
f = supply frequency (Hz)
p = number of stator poles

From equation it can be seen that a change in frequency causes a change in speed. Electric motors can thus be speed regulated by means of varying the frequency

In Europe the frequency is 50 Hz, therefore the speed of electric motors is 6000 divided by the number of poles. At least two poles are required which produces a maximum speed of 3000r/min; 4 poles gives 1500; 6 gives 1000; 8 gives 750 r/min, etc. In the US and some Middle East countries as mentioned, the frequency is 60 Hz.

NPSH

Pump NPSH Requirement (NPSHR)
This figure is the necessary amount of energy required (measured in metres) in the liquid at the pump inlet to overcome the internal losses/resistances within the pump and provide sufficient internal pressure to avoid cavitation. These losses are caused by the flow of liquid through the pump suction passage and the shock loss which occurs at the impeller blade. The NPSHR is calculated by the pump manufacturer and will vary depending on the size and speed of the pump.

Available NPSH (NPSHA)
The available NPSH (NPSHA) is the amount of energy (measured in m) available to the fluid at the pump inlet after the factors from section 1 have been taken into account. For a pump to run the NPSHA value must be greater than the NPSHR value. The NPSHA value will govern the amount of suction lift which may be attained with a given pump or the amount of static suction head which is required above pump suction to ensure correct operation.

Hydraulic power

If a pump were an ideal machine, the required input power to drive the pump would
entirely lift the mass flow rate from one elevation to another. This power is called as the
hydraulic power.

Where
Q= capacity in m3 /h
ρ= liquid density in kg/m3 at pumping temperature
H= differential head in m (meters of liquid column)
g= gravitational acceleration in m/s .

Newtonian Fluid

A fluid is classified as being Newtonian if it conforms to NEWTON’s friction law, i.e. if viscosity remains constant with agitation or varying shear rate (i.e. viscosity is only affected by temperature changes) and its shear rate being proportional to the velocity gradient vertical to the direction of flow.

Of Newtonian fluid characteristics are e.g. the following:

water
oils
gases
mercury
alcohol
petrol

If it is not known whether a fluid is of Newtonian flow characteristics or not, it should be laboratory-tested."

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