Piping & Pumps.pdf

8/18/2019 Piping & Pumps.pdf

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Piping & Pumps DesignPage 1 of 14

Chilled Water Piping

The following organizations in the United States issue codes and standards for piping systems andcomponents

ASME — American Society of Mechanical EngineersASTM — American Society for Testing and MaterialsNFPA — National Fire Protection AssociationBOCA — Building Officials and Code Administrators, InternationalMSS — Manufacturers Standardization Society of the Valve and Fittings Industry, Inc.AWWA — American Water Works Association

Parallel federal specifications also have been developed by government agencies and are used formany public works projects. Chapter IV of ASME Standard B31.9, Building Services Piping, listsapplicable U.S. codes and standards for HVAC piping. In addition, it gives the requirements forthe safe design and construction of piping systems for building heating and air conditioning.

Steel Pipe

Steel pipe is manufactured by several processes. Seamless pipe, made by piercing or extruding,has no longitudinal seam. Other manufacturing methods roll skelp into a cylinder and weld alongitudinal seam. A continuous-weld (CW) furnace butt-welding process forces and joins theedges together at high temperature. An electric current welds the seam in electric resistancewelded (ERW) pipe. ASTM Standards A106 and A53 specify steel pipe. Both standards specify A

and B grades. The A grade has a lower tensile strength and is not widely used.Steel pipe is manufactured with wall thicknesses identified by schedule and weight. Althoughschedule numbers and weight designations are related, they are not constant for all pipe sizes.

Standard weight (STD) and Schedule 40 pipe have the same wall thickness through 10 in. NPS.For 12 in. and larger standard weight pipe, the wall thickness remains constant at 0.375 in., whileSchedule 40 wall thickness increases with each size. A similar equality exists between Extra Strong(XS) and Schedule 80 pipe through 8 in.; above 8 in., XS pipe has a 0.500 in. wall, while Schedule80 increases in wall thickness.

Joints in steel pipe are made by welding or by using threaded, flanged, grooved, or welded outletfittings. Unreinforced welded-in branch connections weaken a main pipeline, and added

reinforcement is necessary, unless the excess wall thickness of both mains and branches issufficient to sustain the pressure.

ASME Standard B31.1, Power Piping, gives formulas for determining whether reinforcement isrequired. Such calculations are seldom needed in HVAC applications because (1) standardweights pipe through 20 in. NPS at 300 psig requires no reinforcement; full-size branchconnections are not recommended; and (2) fittings such as tees and reinforced outlet fittingsprovide inherent reinforcement.


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Copper Tube

Because of their inherent resistance to corrosion and ease of installation, copper and copper alloysare often used in heating, air-conditioning, refrigeration, and water supply installations. There aretwo principal classes of copper tube. ASTM Standard B88 includes Types K, L, M, and DWV forwater and drain service. ASTM Standard B280 specifies air-conditioning and refrigeration (ACR)tube for refrigeration service.

Types K, L, M, and DWV designate descending wall thick nesses for copper tube. All types havethe same outside diameter for corresponding sizes. Usually Tables are used to know theproperties of ASTM B88 copper tube. In the plumbing industry, tube of nominal size

approximates the inside diameter. The heating and refrigeration trades specify copper tube by theoutside diameter (OD). ACR tubing has a different set of wall thick nesses. Types K, L, and M tubemay be hard drawn or annealed (soft) temper.

Copper tubing is joined with soldered or brazed, wrought or cast copper capillary socket-endfittings. Small copper tube is also joined by flare or compression fittings.

Hard-drawn tubing has a higher allowable stress than annealed tubing, but if hard tubing is joined by soldering or brazing, the annealed allowable stress should be used.Brass pipe and copper pipe are also made in steel pipe thick nesses for threading. High cost haseliminated these materials from the market, except for special applications.

The heating and air-conditioning industry generally uses Types L and M tubing, which have

higher internal working pressure ratings than the solder joints used at fittings. Type K may beused with brazed joints for higher pressure-temperature requirements or for direct burial. Type Mshould be used with care where exposed to potential external damage.

Copper and brass should not be used in ammonia refrigerating system The standards givedimensions and pressure ratings for fittings, flanges, and flanged fittings. This data is alsoavailable from manufacturers’ catalogs.

Joining methods

Threading is the most commonly used method for joining small-diameter steel or brass pipe, asdiscussed in ASME Standard B1.20.1. Pipe with a wall thickness less than standard weight shouldnot be threaded. ASME Standard B31.5, Refrigeration Piping, limits the threading for various

refrigerants and pipe sizes.Soldering and Brazing

Copper tube is usually joined by soldering or brazing socket end fittings. Brazing materials melt attemperatures over 1000°F and produce a stronger joint than solder. Health concerns have causedmany jurisdictions to ban solders containing lead or antimony for joining pipe in potable watersystems. Lead-based solders, in particular, must not be used for potable water systems.


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Flared and Compression Joints

Flared and compression fittings can be used to join copper, steel, stainless steel, and aluminumtubing. Properly rated fittings can keep the joints as strong as the tube.

Flanges

Flanges can be used for large pipe and all piping materials. They are commonly used to connect toequipment and valves, and wherever the joint must be opened to permit service or replacement ofcomponents. For steel pipe, flanges are available in pressure ratings to 2500 psig. For welded pipe,weld neck, slip-on, or socket weld connections are available. Thread-on flanges are available forthreaded pipe.

Flanges are generally flat faced or raised face. Flat-faced flanges with full-faced gaskets are mostoften used with cast iron and materials that cannot take high bending loads. Raised-face flangeswith ring gaskets are preferred with steel pipe because they facilitate increasing the sealingpressure on the gasket to help prevent leaks. Other facings, such as O-ring and ring joint, areavailable for special applications.

All flat-faced, raised-face, and lap-joint flanges require a gasket between the mating flangesurfaces. Gaskets are made from rubber, synthetic elastomers, cork, fiber, plastic, teflon, metal,and combinations of these materials. The gasket must be compatible with the flowing media andthe temperatures at which the system operates.

Welding

Steel pipe joints over 2 in. in diameter that have been welded offer the following advantages: Do not age, dry out, or deteriorate as do gasketed joints Can accommodate greater vibration and water hammer and higher temperatures and

pressures than other joints For critical service, can be tested by any of several nondestructive examination (NDE)

methods, such as radiography or ultrasound Provide maximum long-term reliability

Other Joints

Grooved joint systems require that a shallow groove be cut or rolled into the pipe end. These joints can be used with steel, cast iron, ductile-iron, copper, and plastic pipes. A segmented clampengages the grooves, and the seal is provided by a special gasket designed so that internalpressure tightens the seal. Some clamps are designed with clearance between tongue and grooveto accommodate misalignment and thermal movements, and others are designed to limitmovement and provide a rigid system. Manufacturers’ data gives temperature and pressurelimitations.


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Another form of mechanical joint consists of a sleeve slightly larger than the outside diameter ofthe pipe. The pipe ends are inserted into the sleeve, and gaskets are packed into the annular spacebetween the pipe and coupling and held in place by retainer rings. This type of joint can acceptsome axial misalignment, but it must be anchored or otherwise restrained to prevent axial pulloutor lateral movement. Manufacturers provide pressure-temperature data.

Ductile-iron pipe is furnished with a spigot end adapted for a gasket and retainer ring. This joint salso not restrained.

Unions

Unions allow disassembly of threaded pipe systems. Unions are three-part fittings with a matingmachined seat on the two parts that thread onto the pipe ends. A threaded locking ring holds thetwo ends tightly together. A union also allows threaded pipe to be turned at the last jointconnecting two pieces of equipment. Companion flanges (a pair) for small pipe serve the samepurpose.

Plastic piping systems

Nonmetallic pipe is widely used in HVAC and plumbing systems. Plastic is light in weight,generally inexpensive, and corrosion-resistant. Plastic also has a low “C” factor (i.e., its surface isvery smooth), which results in lower pumping power and smaller pipe sizes. The disadvantagesof plastic pipe include the rapid loss of strength at temperatures above ambient and the highcoefficient of linear expansion. The modulus of elasticity of plastics is low, resulting in short

support span distances. Some jurisdictions do not allow certain plastics in buildings because oftoxic products emitted under fire conditions.

Plastic piping materials fall into two main categories—thermoplastic and thermosetting.Thermoplastics melt and are formed by extruding or molding. They are usually used withoutreinforcing filaments. Thermosets are “cured” and cannot be reformed. They are normally usedwith glass fiber reinforcing filaments.

Water Piping Systems Once - thru System Re-circulating System

In a once-thru system water passes thru the equipment only once and is discharged. In re-circulating system water is not discharged, but flows in a repeating circuit from the heatexchanger to the refrigeration equipment and back to the heat exchanger. The re-circulatingsystem may be classified as:

Open System Closed System


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Open System

An open system is one in which the water flows into reservoir open to the atmosphere; Coolingtowers, air washers and storage tanks are the examples of Open System .

Closed System

A Closed System is one in which the flow of water is not exposed to the atmosphere at any point.Closed System some time contains an expansion tank that is open to atmosphere but water areaexposed is insignificant.

Water Return Arrangements

The Re-circulating system is further classified according to water return arrangements. Whenmultiple units of a Closed System are connected, one of the following piping arrangements maybe used:

Reverse Return Piping Reverse Return Header with Direct Return Risers Direct Return Piping

Reverse Return Piping

Reverse return piping is recommended for most closed piping applications. It is often economicaldesign on new constructions. The length of the water circuit thru the supply and return piping is

the same for all units. Since the water circuits are equal for each unit, the major advantage of areverse return system is that it seldom requires balancing. Figure shown is a schematic sketch ofthis system with units piped horizontally and vertically. Reverses return arrangement on an opensystem requires piping that is normally unnecessary, since the same atmospheric conditions existat all open points of the system


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Friction Loss Rate

The design of a water piping system is limited by the friction loss. Systems using city water musthave the piping sized so as to provide the required flow rate at a pressure loss within the pressureavailable at the city main. This pressure or friction loss is to include all losses in the system, ascondenser pressure drop, pipe and fitting losses, static head, and water meter drop. The totalsystem pressure drop must be less than the city main pressure to have design water flow.As per Carrier guide line:

In large air conditioning applications this balance point is often taken as a maximumfriction loss rate 8-10 ft of water per 100 ft of equivalent pipe length.

As per ASHRAE Guide line:


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The Friction Loss rate for Hydronic System should be taken as 1 to 4 Feet/100 feet of PipeEquivalent Length. A Value of 2.5 Feet/100 feet is the mean to which most systems aredesigned .

For 2 Inch and below pipes, Velocity limit is 4 FPS. For above pipes, FLR limit is 4Feet /100.

In the average air conditioning application the installed cost of the water piping exceeds the costof the water pumps and motors. The cost of increasing the pipe size of small pipe to reduce thefriction rate is normally not too great, whereas the installed cost increases rapidly when the size oflarge pipe (approximately 4 in. and larger) is increased. Smaller pipes can be economically sized atlower friction rates (increasing the pipe size) than the larger pipes.

Each system should be analyzed separately to determine the economic balance between first cost(pipe size, pump and motor) and operating cost (pressure drop, pump and motor).

Pipe Length (Equivalent Length )

To determine the friction loss in a water piping system, the engineer must calculate the straightlengths of pipe and evaluate the additional equivalent lengths of pipe due to fittings, valves andother elements in the piping system. Tables give the additional equivalent lengths of pipe for thesevarious components. The straight length of pipe is measured to the centerline of all fittings andvalves. The equivalent length of the components must be added to this straight length of Pipe.

Water Piping Diversity

When the air conditioning load is determined for each exposure of a building, it is assumed thatthe exposure is at peak load. Since the sun load is at a maximum on one exposure at a time, not allof the units on all the exposures require maximum water flow at the same time to handle thecooling load. Units on the same exposure normally require maximum flow at the same time; unitson the adjoining or opposite exposures do not. Therefore, if the individual units are automaticallycontrolled to vary the water quantity, the system water quantity actually required during normaloperation is less than the total water quantity required for the peak design conditions for all theexposures. Good engineering design dictates that the water piping and the pump be sized for thisreduced water quantity.

The water flow to the units must he automatically controlled to compensate for varyingloads.

Diversity may only be applied to piping that supplies units on more than one exposure.

The Figure is a typical illustration of a header layout to which diversity may be applied. In thisillustration the header piping supplies all four exposures. Assuming that the units supplied areautomatically controlled, diversity is applied to the west, south and east exposures only. The lastleg or exposure is never reduced in water quantity or pipe size since it requires full flow at sometime during operation to meet design conditions.


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ASHRAE Recommended "design procedures"

Pump Performance Curves

Performance of a centrifugal pump is commonly shown by a manufacturer’s performance curve .The figure displays the pump power required for a liquid with a specific gravity of 1.0 (water)over a particular range of impeller diameters and flows. The curves are generated from a set ofstandard tests developed by the Hydraulic Institute (1994).The tests are performed by the manufacturer for a given pump volute or casing and severalimpeller diameters, normally from the maximum to the minimum allowable in that volute. Thetests are conducted at a constant impeller speed for various flows. Pump curves represent theaverage results from testing several pumps of identical design under the same conditions. Thecurve is sometimes called the head-capacity curve (H-Q) for the pump. Typically, the dischargehead of the centrifugal pump, sometimes called the total developed head (TDH), is measured infeet of water flowing at a standard temperature and pressure. TDH represents the difference inpressure between the inlet or suction side and the outlet or discharge side of the pump. Thisdischarge head decreases as the flow increases. Motors are selected to be non-overloading at aspecified impeller size and maximum flow to ensure safe motor operation at all flow requirements

The pump characteristic curve may be further described as flat or steep. Sometimes these curvesare described as a normal rising curve (flat), a drooping curve (steep), or a steeply rising curve.The pump curve is considered flat if the pressure at shutoff is about 1.10 to 1.20 times the pressureat the best efficiency point.


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This performance curve, also called a family curve or series, is useful for determining theapproximate size and model required, but the particular pump curve must then be checked toconfirm an accurate selection.Equation shows that pressure drop in a hydronic system (pipe, fittings, and equipment) isapproximately proportional to the square of the flow. Experiments show that pressure drop ismore nearly proportional to between Q 1.85 and Q 1.9.Pump efficiency is determined by comparing the output power to the input power:

Efficiency = Output/Input = Whp/Bhp X 100 %

The pump manufacturer usually plots the efficiencies for a given volute and impeller size on thepump curve to help the designer select the proper pump. The best efficiency point (BEP) is theoptimum efficiency for this pump operation above and below this point is less efficient

Affinity Laws

The centrifugal pump, which imparts a velocity to a fluid and converts the velocity energy topressure energy, can be categorized by a set of relationships called affinity laws. The laws can bedescribed as similarity processes that follow these rules:

Flow varies with the rotating speed

Q2/Q1 = N2/N1 Q2/Q1 = D2/D1 Head varies as the square of rotating speed

H2/H1= (N2/N1) 2 H2/H1 = (D2/D1) 2

BHP Varies as the cube of rotating speed BHP2/BHP1 = (N2/N1) 3


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BHP2/BHP1=(D2/D1) 3 The affinity laws are useful for estimating pump performance at different rotating speeds orimpeller diameters based on a pump with known characteristics. The following two variations canbe analyzed by these relationships:

Selection of Pumps

A substantial amount of data are required to ensure that an adequate, efficient, and reliable pumpis selected for a particular system. The designer should review the following criteria:

1. Design flow2. Pressure drop in highest resistant loop3. Minimum system flow4. System pressure at maximum and

minimum flows

5. Type of Control valves6. Continuous or variable flow7. Number of pumps and standby8. Electric voltage and current9. Electric service and starting limitations

When a centrifugal pump is applied to a piping system, the operating point satisfies both thepump and system curves. As the load changes, control valves change the system curve and theoperating point moves to a new point on the pump curve.Arrangement of Pumps

In a large system, a single pump may not be able to satisfy the full design flow and yet provideboth economical operation at partial loads and a system backup. The designer may need toconsider the following alternative pumping arrangements and control scenarios:

Multiple pumps in parallel or series Standby pump Compound pumping (Primary-secondary pumping) Variable-speed pumping Distributed pumping

Parallel Pumping

When pumps are applied in parallel, each pump operates at the same pressure and provides itsshare of the system flow at that pressure. Generally, pumps of


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equal size are recommended, and the parallel pump curve is established by doubling the flow ofthe single pump curve.

Standby Pump

A backup or standby pump of equal capacity and pressure installed in parallel to the main pumpis recommended to operate during an emergency or to ensure continuous operation when a pumpis taken out of operation for routine service.

Series Pumping

When pumps are operated in series, each pump operates at the same flow rate and provides itsshare of the total pressure at that flow.


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A system curve plotted across the series pump curve shows the operating points for both singleand series pump operation . Note that the single pump can provide up to 80% flow for standbyand at a lower power requirement. Series pump installations are often used in heating and coolingsystems so that both pumps operate during the cooling season to provide maximum flow andhead, while only a single pump operates during the heating season. Note that both parallel andseries pump applications require that the actual pump operating points be used to accuratelydetermine the pumping point. Adding artificial safety factor head, using improper pressure dropcharts, or incorrectly calculating pressure drops may lead to an unwise selection.

Compound Pumping

In larger systems, compound pumping, also known as primary secondary pumping, is oftenemployed to provide system advantages that would not be available with a single pumpingsystem.

In Figure, Pump No. 1 can be referred to as the source or primary pump and Pump No. 2 as theload or secondary pump. The short section of pipe between A and B is called the common pipebecause it is common to both the source and load circuits


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If the flow capacity of Pump No. 1 exceeds that of Pump No. 2, some water will flow downwardin the common pipe. Under this condition, Tee A is a diverting tee, and Tee B becomes a mixingtee.

Again, the temperature of the fluid entering the load is the same as that leaving the source.However, because of the mixing taking place at Point B, the temperature of the water returning tothe source is between the source supply temperature and the load return temperature.

Advantages of compound circuits: They enable the designer to achieve different water temperatures and temperature ranges

in different elements of the system.

They de-couple the circuits hydraulically, thereby making the control, operation, andanalysis of large systems much less complex. Hydraulic de-coupling also preventsunwanted series or parallel operation.

Variable-Speed Pumping

In a variable-speed pumping arrangement, constant flow pump(s) re-circulate the chiller or boilersource in a primary source loop, and a variable-speed distribution pump located at the sourceplant draws flow from the source loop and distributes to the load terminals.

The speed of the distribution pump is determined by a controller measuring differential pressureacross the supply-return mains or across selected critical zones. Two-way control valves areinstalled in the load terminal return branch to vary the flow required in the load

Distributed PumpingIn a variable-speed distributed pumping arrangement, constant flow pump(s) re-circulate thechiller or boiler source in a primary source loop, and a variable-speed zone or building pumpdraws flow from the source loop and distributes to the zone load terminals. The speed of the zoneor building distribution pump is determined by a controller measuring zone differential pressureacross supply-return mains or across selected critical zones. Two-way control valves in the loadterminal return branch vary the flow required in the load.


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