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Learn Basic of Piping Engineering: Chapter-3

Posted by Antony Thomas at Monday, February 28, 2011



CHAPTER-3
FLUID

INTRODUCTION:
It can be
·         A gas
·          A liquid
·          A mixture of gas or liquid
·          A suspension of small solid particles inside a liquid.

BASIC PROPERTIES OF CONVEYED FLUIDS:
  • Fluid type
  • Flow rate
  • Pressure
  • Temperature


FACTORS DEPENDING UPON FLUID TYPE:

1. MATERIAL:
a. Non corrosive fluids: Services where impurities are accepted
Example:
Industrial water lines (cooling water)
Steam
Lube oil return / before filter lines
Air lines
Vents and drains
Material:
Carbon Steel
Low Alloy Steel (High T)
b. Corrosive fluids: Services where impurities are not accepted
Example:
Demineralized water
Lube oil after filters
Fuel gas / oil
Sea water (water containing Chlorine)
Material:
Stainless Steel
No Iron (Fe)
Copper/Nickel Alloys (Cu-Ni)
c. Aggressive Chemicals
Example
Strong Acids / Bases
Material:
Plastic: PVC – TEFLON – PE
Rubber: NBR, Viton
Composites: RESIN GLASS
2. CORROSION ALLOWANCES:
Thickness of the pipe increases with respect to corrosion. Typical corrosion allowance for water is 3mm.

3. TYPE OF JOINT:
Dangerous fluids are conveyed in fully welded pipes, were leaks are not accepted.

4. TESTING AND EXAMINATION:
For Dangerous Fluids 100% of joints are likely to be X-Ray examined


FACTORS DEPENDING UPON FLOWRATE:
1. DIAMETER:
For a given flow rate:
Small diameter means higher velocity of the conveyed fluid.
Big diameter means slower velocity of the conveyed fluid.

Velocity of fluids in pipelines affects:
Pressure losses along the pipeline.
Pressure losses are proportional to the square velocity (v2).
Vibration of the pipeline.

Usual velocities of fluids inside pipelines are:
Gas: 20 m/s - max. 40 / 50 m/sec.
Liquid: 2 to 4 m/s - max. 10 m/sec.


FACTORS DEPENDING UPON PRESSURE:
1. WALL THICKNESS CALCULATION
2. TYPE OF JOINT
Low pressure pipelines can be threaded or socket welded
High Pressure pipelines are Butt Welded
3. TESTING AND EXAMINATION:
Non process Pipelines (For Example Vents and drain lines) may even have no tests at all Low Pressure Pipelines can undergo only the Hydraulic Test For intermediate pressures a 10% to 50% of joints must be examined with X-rays High Pressure Pipelines are usually 100% X-ray examined.

FACTORS DEPENDING UPON TEMPERATURE:
1. MATERIAL:
Steel for High Temperature (Low Alloy Steel Creep Resistant)
2. WALL THICKNESS CALCULATION
3. THERMAL INSULATION:
T>60°C Insulation for Personnel Protection is mandatory for all pipeline parts that can be reached by hands.
4. STRESS ANALYSIS:
Hot Lines must be routed properly. Provisions shall be taken so that when temperature rises from ambient to Operating Temperature the thermal expansion of Pipelines does not generate stresses too high for the pipes to withstand.

FLUID SERVICE CATEGORIES:
B31.3 recognizes the following fluid service categories and a special design consideration based on pressure. It is the owner’s responsibility to specify the fluid service category for each piping system. With the fluid service category known, then the designer can make material and component selection, as well as employ the code required fabrication and inspection requirements based on the selected fluid category. These fluid categories and pressure concern are:

1. Normal fluid service
2. Category D fluid service
3. Category M fluid service
4. High pressure piping
5. Severe cyclic conditions

Category D Fluid Service is defined as all fluid services that are:
· Nonflammable
· Nontoxic
· Not damaging to human tissues
· The design gage pressure does not exceed 150 psig
· The design temperature is from -20° f to 366° f is the saturated temperature of  steam at 150psig

Category M Fluid Service is defined as a service in which a single exposure to a very small quantity of toxic fluid can produce serious irreversible harm to person on breathing or bodily contact, even when prompt restorative are taken.

The Normal Fluid Service is defined as all other fluid services that are not category D and category M

High Pressure Piping Service is defined as that in which the pressure is in excess of that allowed by the ASME B16.5 2500 flange class ratings.

PIPING SYSTEM CORROSION:

General or Uniform Corrosion:
Uniform metal loss. May be combined with erosion if high velocity fluids, or moving fluids containing abrasives.

Pitting Corrosion:
Localized metal loss randomly located on material surface.
Occurs most often in stagnant areas or areas of low flow velocity.

Galvanic Corrosion:
Occurs when two dissimilar metals contacts each other in corrosive electrolytic
environment.

Concentration Cell Corrosion:
Occurs when different concentration of either a corrosive fluid or dissolved oxygen contacts areas of same metal.

Crevice Corrosion:
Localized corrosion similar to pitting.
Occurs at places such as gaskets, lap joint, and bolts where crevice exists.

SEE ALSO:

Learn Basic of Piping Engineering: Chapter-2



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