Vessel Design and SpecificationProcess Engineering

By Travis Abrams

Program ObjectivesProgram Objectives

• Describe the design criteria for a vessel

• Explain how to size the vessel

• Show how to create the specification

Design CriteriaDesign Criteria

• Surge time• Hold up time• Total capacity• Pressure of storage• Control response time• Does it separate vapor from liquid?• … or liquid from liquid? • … or both?

Codes and StandardsCodes and Standards• Codes and Standards

• ASME

• For Vessels– Section I Boilers

» Rules for construction of power Boiler

– Section VIII Unfired Pressure Vessels

» Rules for construction of pressure vessels

• For Piping– B31.1 Power Piping

» Boiler and power piping

– B31.3 Process Piping

» Chemical plant and refinery piping

Look here to get the official design criteria especially for detailed mechanical specifications.

To The Codes!

ASME Section VIII

ASME B31.3

Pressure and Service determines the type of Tank

Pressure and Service determines the type of Tank

1. Flat Roof 2. Floating roof3. Cone roof4. Domed Roof5. Spherical Pressure Vessel6. Cylinders with heads

PIP has a tank selection guide

PIP Tank Selection Guide

Tank Design PressureTank Design Pressure

• Cone roof- 8” H2O and 2.5 “ H2O

Domed Roof Design PressureDomed Roof Design Pressure

• 0.5 to 15 psig

Floating Roof Tanks

From PIP Tank Selection guide:

Floating Roof Tanks

From PIP Tank Selection guide:

Pipeline Slugcatchers

• Conventional

•Finger Type-by a Slugcatcher Manufacturer

Both sizes are determined by the vapor rate and the slug size coming from the pipeline during pigging operations.

Flare KO Drums• Design by RP-521 Standards

Steam Condensate Pots

• Size based on pump requirements or Control Valve response time.

Reactors

• Usually designed by the client or licensor

• Sometimes it designed by the catalyst supplier.

• Internals can be quite elaborate and is a design course all by itself.

SurgeSurge

SurgeSurge

SurgeSurge

HoldupHoldup

HoldupHoldup

HoldupHoldup

CapacityCapacity• Small Capacity: Day tanks are sized for 24

hrs of operation

• Large capacity: Refinery storage is sized based days or months.

Designing Drums• Horizontal? If there is more liquid than vapor, chances

are you will choose a horizontal drum.

An example would be a propane surge drum for a propane vaporizer.

• Or Vertical?

An example for a vertical drum is a Compressor suction drum-usually very little liquidIs present.

Vertical Drum Theory

,2

1

V

VLT KU

UT is the terminal velocity of a given size droplette that is flowing with the vapor as it goes up the drum

VD

VLP

T C

gDU

3

)(4

Simplified to a Sauder-Brown type equation:

K ranges from 0.2 to 0.35

Horizantal Drum Theory

• “For horizontal separator design, the subsequent design procedures use a “droplet settling approach” similar to the API procedure (6) which does not require empirical modification of the “K” value for vertical settlers.” –from Design Two-Phase Separators Within the Right Limits

Process Data• Data usually comes from a simulation• For some drums, there is no liquid-

therefore you must look upstream for possible liquid sources possibly from a process upset.

• Example: Liquid carry-over from TEG unit to compressor KO drum.

3 Phase Separators3 Phase Separators• Adds a dimension of

complication-usually a water boot and a weir internal to the vessel.

• Residence times determined by the particle size of the water droplet or HC droplet.

• Coalescing pads will shorten the vessel length-this is a good time to consult a coalescer vender. Usually spec is % removal of a certain size droplette or (easier to understand) PPMvol of free water in oil or oil in water.

3-Phase3-Phase

Mechanical separation works very well!

For High-viscosity liquids, such as caustic, it is the only way to make a good separation.

Design Guide•Our Vessel Design guide is for non client specified vessel sizing. Often clients have their own methods and standards•We have at least five programs to choose from!• Only the Worley programs (found on WOW) have been officially approved.• Kishore Joshi’s program will probably be the front runner as the official program in our office.•Worley has a 3 phase program (found in WOW)• So far, our programs and guides only address two-phase separation.

To The Standards!

Steps to Sizing the Drum

1. Get process data.

2. Decide Horizontal or vertical

3. Read the appropriate design guide.

4. Decide if a mesh pad or vane pack is to used or not (usually a client preference- or an economical choice)

5. Calculate the K factor

Tangent or Seam?

Vol surge

Vol hold

12" min

24" min

D

1D noz

6" min

12" min

12" min

Highest Liquid Level

NLL

12" min

or (Mod-Nod)/2

LLL

12" min6" min

Vapor Outlet

Inlet Nozzle

Liquid Outlet

Holdup Surge5 2

5 2

10 2

10 25 2

10 3

8 42 1

to storage 5 2

to onplot processing

5 5

to offplot processing

10 10

5 35 515 5Steam drum(s)

(Horizontal vessel)

Refrigerant flash drumNet Product

Condensate potsBoot

Feed to a furnaceFeed to a low head charge pumpFeed to a high head multistage pumpKnock-out drums

ServiceColumn feeding to another column (Column liquid reservoir)Feed to a distillation columnFeed to a string of columns

Design, continued

5. Choose a surge time, hold up time and a time to alarm-either to HHLL or LLLL both. Our design guide has a table- or use the client preferences.

Vessel Height or Skirt Height

• Pump NPSH (from Pump hydraulic calculation

• Piping requirements (may need help from piping)

• Downstream pressure requirements (gravity flow)

Based on the cost estimates, this table of vessel ID was put together by the Mechanical group:

•Once you rough in the sizes-go back and change the ID to the most economical size based on the table provided by mechanical.

Design-continued

• Next check the L/D ratio: Operating Pressure L/D ratio

Below 250 psig 1.5 – 3.0

250 – 500 psig 3.0 – 4.0

Above 500 psig 4.0 – 6.0

•Readjust the ID until both economical ID and L/D criteria is met.•Another thing to watch out for- often the client wants horizontal drum NLL to be centerline-you must adjust the ID and the L/D ratio to get it to match up

Design Continued

• Calculate the Design Pressure and temperature. Use the Job-approved method or the method Spelled out in PTD-DGS-102.

• Next-size the lines and get the nozzle size guidelines out for vents, drains, instruments, steam out.

What are some typical Nozzles?• Inlet (possible more than one)• Outlet (possibly more than one)• Drain• Vent to Flare• Vent to Atmosphere• Steam out• Manway- or two• Relief Valve• Level bridle• Separate level instrument• Pressure gauge• Temperature gauge• Air mover- usually 8”

Don’t forget the vortex breaker!

Some client have a specific nozzle tag system-understand the system before you start, so you don’t have to rework your nozzle schedule.

N1

Flange Rating from GPSA

It’s Time to Make the Vessel drawing!

• Use Smartsketch to make the vessel drawing.

• Details that are a must:1. ID dimension and T/T or S/S

dimension.2. Nozzle placement and dimensions

both in the vertical and horizontal-unless it does not matter and it is a piping matter.

3. Level callouts and dimensional height from the bottom or tangent line.

4. Internal details (example: mesh pad) and placement-including inlet boxes or internal piping.

5. Skirt height or vessel height above grade.

Nozzle Location

• Usually nozzles centerline should be at least one pipe diameter from a weld seam.

Inlet Boxes• Inlet boxes divert

the inlet flow to change the incoming flow direction

• Types of inlet diverters:

• Boxes• Pipes with elbow

fitting

Special Inlet Boxes

• Shell SchoepentoeterTM

Tangential entry nozzle with wear pad

Converting the Sketch

• Not intuitive in Word (excel is better)

• I like this method:1. Highlight Drawing

2. Hit copy (cntrl C)

3. Paste into excel or

1) Save as *.cgm and “insert Picture”-the drawback is the text usually does not come out right

Material Specs• The vessel materials

are generally specified by the material diagram ( a special marked up PFD) that is produced as a job document. Specific materials of construction should be known and approved by the lead Process engineer or specified by the client or process licensor.

• Usually the process engineer does not have to give a detailed material specification but rather “CS” for carbon steel or “316 SS” is usually enough detail for the mechanical vessel engineer.

Supplemental Data

• Some information that will be needed by others will need to be put in the notes section:

Example: Vane pack process data and specification

Finished Package

• The calculation cover sheet (includes checking signatures)

• Spec Sheet• Calculations• All supporting

documentation