Process Design CEN 574 Spring 2004 Chemical Engineering and Materials Science Syracuse University Piping and Pumping Process Design CEN 574 Spring 2004

Outline NPSH and cavitation Regulation of flow Pipe routing Optimum pipe diameter Pressure drop through piping Piping costs Pump types and characteristics Pump curves NPSH and cavitation Regulation of flow Pump installation design

Piping and Pumping Learning Objectives At the end of this section, you should be able to… Draw a three dimensional pipe routing with layout and plan views. Calculate the optimum pipe diameter for an application. Calculate the pressure drop through a length of pipe with associated valves. Estimate the cost of a piping run including installation, insulation, and hangars.

List the types of pumps, their characteristics, and select an appropriate type for a specified application. Draw the typical flow control loop for a centrifugal pump on a P&ID. Describe the features of a pump curve. Use a pump curve to select an appropriate pump and impellor size for an application. Predict the outcome from a pump impellor change. Define cavitation and the pressure profile within a centrifugal pump. Calculate the required NPSH for a given pump installation. Identify the appropriate steps to design a pump installation.

References Appendix III.3 (pg 642-46) in Seider et al., Process Design Principals (our text for this class). Chapter 12 in Turton et al., Analysis, Synthesis, and Design of Chemical Processes. Chapter 13 in Peters and Timmerhaus, Plant Design and Economics for Chemical Engineers. Chapter 8 in McCabe, Smith and Harriott, Unit Operations of Chemical Engineering.

Pipe Routing The following figures show a layout (looking from the top) and plan (looking from the side) view of vessels. We want to rout pipe from the feed tank to the reactor.

Plan View piping chase reactor steam header feed tank 40 ft 60 ft

Layout View: Looking Down steam header 40 ft feed tank piping chase 45 ft 30 ft reactor 10 ft reactor 50 ft 35 ft

Plan View piping chase reactor = out = in steam header feed tank 40 ft

Layout View steam header feed tank reactor 85 ft 30 ft 20 ft 35 ft

Pipe Routing Exercise Form groups of two. Draw a three dimensional routing for pipe from the steam header to the feed tank on both the plan view and the layout view.

Size the Pump Determine optimum pipe size. Determine pressure drop through pipe run. 200 ft globe valve check valve 150 ft 100 gpm

Optimum Pipe Diameter The optimum pipe diameter gives the least total cost for annual pumping power and fixed costs. As D , fixed costs , but pumping power costs .  

Optimum Pipe Diameter Total Cost Optimum Annualized Capital Cost Pumping Power Cost

Example Two methods to determine the optimum diameter: Velocity guidelines and Nomograph. Example: What is the optimum pipe diameter for 100 gpm water.

Using Velocity Guidelines Velocity = 3-10 ft/s = flow rate/area Given a flow rate (100 gpm), solve for area. Area = (/4)D2, solve for optimum D. Optimum pipe diameter = 2.6-3.6 in. Select standard size, nominal 3 in. pipe.

3.3 in optimum diameter Nomograph -Convert gpm to cfm  13.4 cfm. -Find cfm on left axis. -Find density (62 lb/ft3) on right axis. -Draw a line between points. -Read optimum diameter from middle axis. 3.3 in optimum diameter

Practice Problem Find the optimum pipe diameter for 100 ft3 of air at 40 psig/min. A = (s/50ft)(min/60 s)(100 ft3/min) = 0.033 ft2 0.033 ft2 = 3.14d2/4 d = 2.47 in

Piping Guidelines Slope to drains. Add cleanouts (Ts at elbows) frequently. Add flanges around valves for maintenance. Use screwed fitting only for 1.5 in or less piping. Schedule 40 most common.

Calculating the Pressure Drop through a Pipe Run Use the article Estimating pipeline head loss from Chemical Processing (pg 9-12). P = (/144)(Z+[v22-v12]/2g+hL) Typically neglect velocity differences for subsonic velocities. hL = head loss due to 1) friction in pipe, and 2) valves and fittings. hL(friction) = c1fLq2/d5

c1 = conversion constant from Table 1 = 0.0311. f = friction factor from Table 6 = 0.018. L = length of pipe = 200 ft + 150 ft = 350 ft. q = flow rate = 100 gpm. d = actual pipe diameter of 3” nominal from Table 8 = 3.068 in . hL due to friction = 7.2 ft of liquid head

Loss Due to Fittings K= 0.5 entrance K = 1.0 exit K=f(L/d)=(0.018)(20) flow through tee K=3[(0.018)(14)] elbows K=0.018(340) globe K=0.018(600) check valve Sum K = 19.5

hLsum=7.2 + 5.7 ft of liquid head loss Using Bernoulli Equation hL due to fittings = c3Ksumq2/d4 = 5.7 ft of liquid head loss due to fittings. hLsum=7.2 + 5.7 ft of liquid head loss Using Bernoulli Equation P = (/144)(Z+[v22-v12]/2g+hLsum) P = ( /144)(150+0+12.9)= 70.1 psi due mostly to elevation. Use P to size pump. elevation velocity friction and fittings

Find the Pressure Drop check valve 400 gpm water 4 in pipe 400 ft

Estimating Pipe Costs Use charts from Peters and Timmerhaus. Pipe Fittings (T, elbow, etc.) Valves Insulation Hangars Installation

Note: not 2003 $ $/linear ft

Pumps – Moving Liquids Centrifugal Positive displacement Reciprocating: fluid chamber stationary, check valves Rotary: fluid chamber moves

Centrifugal Pumps

                       

Centrifugal Pump Impeller

Positive Displacement: Reciprocating Piston: up to 50 atm Plunger: up to 1,500 atm Diaphragm: up to 100 atm, ideal for corrosive fluids Efficiency 40-50% for small pumps, 70-90% for large pumps

Positive Displacement: Reciprocating (plunger)

Positive Displacement: Rotary Gear, lobe, screw, cam, vane For viscous fluids up to 200 atm Very close tolerances

Positive Displacement: Rotary

Comparisons: Centrifugal larger flow rates not self priming discharge dependent of downstream pressure drop down stream discharge can be closed without damage uniform pressure without pulsation direct motor drive less maintenance wide variety of fluids

Comparisons: Positive Displacement smaller flow rates higher pressures self priming discharge flow rate independent of pressure – utilized for metering of fluids down stream discharge cannot be closed without damage – bypass with relief valve required pulsating flow gear box required (lower speeds) higher maintenance

Centrifugal Pumps Advantages Disadvantages simple and cheap uniform pressure, without shock or pulsation direct coupling to motor discharge line may be closed can handle liquid with large amounts of solids no close metal-to-metal fits no valves involved in pump operation maintenance costs are lower Disadvantages cannot be operated at high discharge pressures must be primed maximum efficiency holds for a narrow range of operating conditions cannot handle viscous fluids efficiently

Moving Gases Compression ratio = Pout/Pin Fans: large volumes, small discharge pressure Blowers: compression ratio 3-4, usually not cooled Compressors: compression ratio >10, usually cooled. Centrifugal (often multistage) Positive displacement

Fan Impellers

Two-lobe Blower

Reciprocating Compressor

Centrifugal Pump Symbols

Pump Curves For a given pump The pressure produced at a given flow rate increases with increasing impeller diameter. Low flow rates at high head, high flow rates at high head. Head is sensitive to flow rate at high flow rates. Head insensitive to flow rate at lower flow rates.

Pump Curve - used to determine which pump to purchase Pump Curve - used to determine which pump to purchase. - provided by the manufacturer.

Pump Curve Low flow at high head Pressure increases with diameter Head sensitive to flow at high flow rates

NPSH and Cavitation NPSH = Net Positive Suction Head Frictional losses at the entrance to the pump cause the liquid pressure to drop upon entering the pump. If the the feed is saturated, a reduction in pressure will result in vaporization of the liquid. Vaporization = bubbles, large volume changes, damage to the pump (noise and corrosion).

Pressure Profile in the Pump

NPSH To install a pump, the actual NPSH must be equal to or greater than the required NPSH, which is supplied by the manufacturer. Typically, NPSH required for small pumps is 2-4 psi, and for large pumps is 22 psi. To calculate actual NPSH… NPSHactual= Pinlet-P* (vapor pressure) Pinlet = P(top of tank, atmospheric) + gh - 2fLeqV2/D

What if NPSHactual < NPSHrequired? INCREASE NPSHactual cool liquid at pump inlet (T decreases, P* decreases) increase static head (height of liquid in feed tank) increase feed diameter (reduces velocity, reduces frictional losses) (standard practice)

Regulating Flow from Centrifugal Pumps Usually speed controlled motors are not provided on centrifugal pumps, the flow rate is changed by adjusting the downstream pressure drop (see pump curve). Typical installation includes a flow meter, flow control valve (pneumatic), and a control loop.

Typical Installation operator set-point FT FC FV

Designing Pump Installations use existing pump vendor, note spare parts the plant already stocks. select desired operating flow rate, maximum flow rate. calculate pressure drop through discharge piping, fittings, instrumentation (note if flow control is desired need to use pressure drop with control valve 50% open).

add safety factor to calculated head – 10 psig spec pump for 20 psig, 150 psig spec pump for 200 psig. using head and flow rate, select impeller that gives efficient operation in region of operating flow rate. vertical location of pump compared to level of influent tank (NPSH). if want to control flow rate – spec and order flow meter and flow control valve also.