Basis of Calculation: Find P at a using hpg. Find P at b by considering frictional loss. Find P at c with known P at d. The head is then known, which is under open valve situation. Head increases approximately 10% if the valve is fully close (shut-in head).

2. Saturated temperature, ST is approximately sqrt(sqrt(P))*100.

3.

AB represents the system friction losses. The solid curves represent the head loss under the condition of fully open valve. The valve can be controlled to apply head losses to reach duty point C. Therefore the system maximum and minimum flow must lie within the flow range.

Problem services

Tank, P1

Pumpa

b

c

dTank, P2

S1

S2

Fig 1: Pumping System Schematic

System static head, Fixed head losses = (S2-S1) + (H2-H1) – see fig 1.

D

C

B

Head, m

Q, m3/sQc Qb

A

Fig 2: Pump & System characteristics

1. Hot services where temperature is above 175 degree celcius2. Low NPSH services which take suction from a drum where the fluid is in

vapor/liquid equilibrium3. Cryogenic services. The concern is to make sure that the pump doesn’t vapor bind

and that the seal environment is controlled4. Vacuum services. The seal environment must be maintained above atmospheric

pressure during operation to keep the seal face lubricated. Proper venting of the pump is also a concern.

5. Low flash point/toxic services. Seal design is a critical to prevent leak from becoming a fire or hygiene hazard

6. Auto-start services. The main concerns are water hammer from on-off valves located well downstream of the pump and reliable remote starting capability

7. Emergency services. The system design must be reviewed to verify the pumps will operate under such conditions

8. Viscous services. Warm-up facilities are important to make sure that the liquid in the pump are at suitable viscosity before the pump is started.

Pump designs to avoid

1. Closed coupled horizontal pumps2. Two-stage overhung pumps3. Double suction overhung pumps

Care should be exercise for using the following pumps

1. Vertical multistage pumps2. Close coupled vertical pumps3. Two stage, face to face impeller pumps

Pump Factors to be considered

1. Material Stress in pump nozzles2. Distortion of internal moving parts affecting clearances in precision parts3. Stresses in pump hold-down bolts4. Distortion in pump support and base plate resulting in driver coupling

misalignment

Suction Specific Speed of pump

Suction Specific Speed is an important consideration when purchasing or analyzing centrifugal pumps. There is a direct correlation between this parameter, pump reliability and maintenance expenses. Centrifugal pumps with SSS larger than 11,000 failed at a frequency nearly twice that with SSS less than 11,000. SSS is defined only at the pump’s best efficiency handbook.

The NPSHr to prevent pressure pulsation, and 3% drop in pump total head are identical for pump with/without backflow recirculator at best efficiency point. However, with backflow recirculator, the pump requires less NPSHr as flow rates decreases.

Process engineering design criteria

Fluid characterizationDesign safety marginMeteorological dataDesign pressure & design temperature philosophyEquipment sizing philosophyLine sizing philosophyUtility systemHydrate inhibitionChemical consumptionFlare and blowdown philosophyDrainage philosophyHeating and cooling medium philosophyIsolation philosophySparing philosophy

Suction Problem

NPSHa and SSS are the main problems. NPSHa is influenced by suction line configuration and the specific product being pumped. SSS influenced by relationship between actual flow rate and Best efficiency point flow rate.

Suppression pressure refers to the threshold pressure required for a vaporized liquid to implode.

Designing Centrifugal Pump System

Normal Flow refers to steady state design, Rated Flow is to provide some measures of contingencies, and Alternate Flow is to assure flexibility of the design for any other operating cases.

Assumptions for Darcy Equation:

Only flow rate defers, temperature and composition remains the sameFlow is turbulent throughoutFriction factor remains constant over the range of flow rate.Frictional drop in equipment treated the same manner as in piping.

Darcy Equation delP2=delP1*(V2/V1) ^2

A piping section from which piping branches off is referred to as a common.

Calculation Procedure:For each operating case, determine the flow rates in each branch, the commons, and at the pump to correct the frictional delP. Rate Flow Rate, Qr = Qn*(Qb*sp.gv)Calculate the rated-case pump suction pressure, using the LLL level as the suction static height.Calculate the NPSHCalculate the discharge. The designer must allow a minimal control-valve differential for each branch. Typically, this differential is based on a percentage of the frictional pressure drop (ie, 10-20%) or an absolute minimum (5-10psi).

Pump discharge piping is generally one size larger than the pump discharge nozzle size.When pumping saturated liquid, if vaporization occurs after equipment, the friction resistance in the pipe would increase and this potion of pipe should be calculated as two-phase flow. If two-phase flow is expected between two equipments, they should be placed side by side to keep the piping short.

Flow sensing controls provide the most stable control. Pressure control can have very large swing when operating in flat area of the pump curve. Both temperature and level sensing control can lead to pump running at end-of-curve condition during failure modes.

Selection of mechanical seals

Selection of an unbalanced or balanced seal is determined by pressure in seal chamber. If b (the ratio of hydraulic closing area to seal face area) is > 100%, it is unbalanced.

Selection of rotating or stationary seal is determined by speed of pump shaft. If shaft speed is >25.4 m/s, use stationary seal. Higher speed applications require a rotating mating ring. Stationary seal should be considered for all split case pumps.

Selection of single or multiple spring seal head construction is determined by the space limits and the liquid sealed. Single spring seals are often used with bellow seals to load the seal faces. Multiple spring seals require a shorter axial space. Most multiple spring designs are used with O-rings or hedges as secondary seals.

Pusher-type seals apply to seals that use an O-ring, wedge, or V-ring. Nonpusher applies to seals that use half-, full-, and multiple-convolution bellows as a secondary seal.

Compressor

Compressor can be designed quite accurately. It is one the most expensive equipment in the plant and has long delivery time. The flows shown on the compressor schedule are normal flows. The compressor shall be rated for 105% of these normal flows. The maximum efficiency should be at normal flow.

Safety Valve

In a process component with liquid & gas section, PSV or vent should be installed to relieve pressure from the gas section.

Critical line:for process, normally those PSV, BDV inlet outlet line are critical (depend)for piping... normally line size >6" they consider critical.(depend)high pressure, toxic,where any process upset can trip the whole process or plant.

Valve Selection

Ball Valve – Suitable for most manual on-off hydrocarbon or utilities service when operating temperatures are between -20F and 180F.

Gate valves – Suitable for most on-off, non-vibrating hydrocarbon or utilities service for all temperature range. Gate vales have better torque characteristics than ball or plug valves but do not have the easy operability of quarter turn action. Reverse acting slab gate valves are suitable for automatic shutdown service.

Plug valves – Similar to ball valves. It is based on lubricated or non-lubricated designs.

Butterfly valves – Suitable for coarse throttling and other applications where a tight shutoff is not required. They are not suitable for primary block valve for vessels.

Globe valves – Suitable when good throttling control is required (e.g. in bypass service around control valves)

Diaphragm (Bladder) Valves – Primarily used for low pressure water (200psig or less) service. They are suitable for systems containing appreciable sand or other solids

Needle Valves – Miniature globe valves. Used for instrument lines. The small passageways through needle valves are easily plugged, and this should be considered in their use.

Check Valves – A full opening swing check is suitable for most non-pulsating applications and it can be used for vertical run pipe (upward & not downward)