7/31/2019 Tank Roof Design - Constructability

1/4

Rule-of-thumb for tank roof plate thickness calculation says:

P x R Where:t =

( S x J x M )

+ CA

P.- Design (Internal) PressureR.- Roof Normal radius

S.- Material Allowable Stress (Carbon Steel)

J.- Join efficient

M.- Temperature reduction factor

CA.- Corrosion allowance

If we are going to use standard Carbon Steel as the tank construction material (we will)and we want to use minimum commercial plate thicknesses for instance: 1/4 inch- to

conform tank roof (and we want), there are two variables we can play with to make that

happen:

Roof Normal radius (R); and Design Internal- Pressure (P)

Roof Normal radius

In order to make the Normal Radius (R) smaller, we can increase the inclination of theroof. However if we increase that inclination too much we will be introducing the

requirement of a top access platform see figure 1- with the consequent increase inconstructability issues (like not being able to use automatic welding machine due to the

roof inclination) and cost of the tank (additional steel for platforms and roof are in

excess)

R1R2

12

0,75 - 2

12

> 2

No roof platformrequired

Roof platform

required

R1 > R2

Fig. 1 Impact of blanketing gas pressure on Atm. Tanks Constructability

7/31/2019 Tank Roof Design - Constructability

2/4

7/31/2019 Tank Roof Design - Constructability

3/4

Having said all that, what Y&V is trying to accomplish is to get included into theConstructability Check List the following issue:

Description

Blanketing pressure for atmospheric tanks should be set to a value as low as possible inorder to avoid increasing the thickness and the inclination of the tank roof as well as the

requirement for top-tank access platforms. Interdisciplinary coordination (Control

System/Process/Mechanical) is required when setting blanketing pressure for atmospherictanks.

Impact On

HSE (hazardous working conditions due to tank roof inclination) Schedule (Not using Automatic Welding Machines due to tank roof inclination

will introduce delays and lower productivity; or, using plates thicker than inch

FULL TANK PROTECTION SYSTEM

Working

Band

ERV Relieving Full Flow

ERV Setting

PVRV Relieving Full Flow

PVRV Setting

Blanketing System Closes

Atmospheric Pressure

Vacuum Valve Setting

7.5

6.0

4.0

0.50

-1.0

Design Tank Pressure (MAWP)

15%9.0

0

25%

50%

Blanketing System Full Open0.4315%

Design Tank Pressure (MAWV)

Droop>

Fig. 3 Atm. Tanks Pressure sets rule-of-thumb

7/31/2019 Tank Roof Design - Constructability

4/4

Fig. 4 Atm. Tanks Pressure setsin a previous job