Emergency Release of Internal Pressures

from Aboveground Storage

TanksBy George L. Morovich

TANK and ENVIRONMENTAL

TECHNOLOGIES, INC.13 Flower Tuft Ct.

The Woodlands, Texas 77380

PH: +1-979-247-4812

GeorgeL@Morovich.com

Emergency pressure release alternatives. The causes, frequency and risks of emergency internal pressure incidents.

Methods for providing emergency release of internal pressure from within aboveground storage tanks are desired as a safety precaution to prevent shell uplift.

Shell uplift may cause failure of the bottom to shell joint along with damage to piping, sumps and appurtenances; thereby, resulting in a release of a tank’s contents.

There currently are three basic approaches for emergency release of internal

pressure.

1. Emergency venting per API 2000

2. Weak roof to shell seam (Frangible roof)

3. Floating roof with open circulation venting

Emergency Venting

Per API 2000 applies only for relief of internal pressures resulting from exposure to an external fire.

Frangible Roof Joint (weak roof seam)Based on completed research (allowing minimal shell uplift), this approach is not reliable for tanks below 50’ diameter.

Current API research investigated relative strength of roof-to-shell and bottom-to-shell joints along with allowable uplift.

Results of research expected to allow frangible joint down to 30’ diameter (with improved bottom joints and consideration of tank uplift, similar to Appendix E earthquake design for uplift requiting flexibility in piping and attachments).

Frangible Roof Joint (weak roof seam)1. Applicable to Cone Roofs only, based on specified

roof slope, top of shell section detail and maximum weld size.

2. Subject to workmanship (such as excessive welds),

3. Subject to changes that weaken the relative strength of the bottom joint (such as bottom settlement and corrosion).

4. Incident results in roof damage (typically requiring removal from service).

5. Not suitable for certain incidents (such as overfill).

Floating roof tanks with open venting or other means to avoid combustible mixture

Considered exempt from the condition that API 2000 provides protection for.

1) The floating roof minimizes exposed product.

2) Circulation vent area will exceed the area required in API 2000.

3) Circulation vents or other means used to minimize the potential hazard of combustible mixtures from developing within the tank.

Domed External Floating-Roof Tank

Internal Floating-Roof Tank

Internal Floating-Roof Tank

Fixed roofFloating roof

Internal Floating Roof Tank --- Internal deflagration reported due to lightening strike during filling

Over Pressure Event

Over Pressure Event

Areas of bottom plate lifting.

Area of top shell course damage.

TYPES OF EMERGENCY INTERNAL PRESSURE EVENTSOverfillSteam ReleaseGas InjectionProduct ReactionsExternal fire exposureInternal deflagrationEarthquakeOtherOf course other causes of roof / shell failure occur,such as incidents involving excessive wind orexposure to an external deflagration.

Overfill incidents appear to be the most common and preventable cause of emergency pressure related damage to tanks. Minimum emergency vents or a frangible joint cannot be expected to relieve pressure prior to uplift. Product level control is the best means to avoid this type of incident. Tank design with low profile floating roofs and rim vents can avoid damage.

Aluminum dome peripheral venting facilitates overfill with minimal damage or no damage to the dome roof. Any resulting damage to the dome roof can typically repaired without removal of the tank from service. Overfill slots are not desired due to reduced operating capacity.

OVERFILL in a cone roof tank causes damage to frangible roof.

Steam Release is primarily a concern on heated tanks.

Gas Injection

These types of incidents can also damage the internal floating deck; however, if the tank is equipped with circulation venting the damage is typically limited to the floating deck.

Over Pressure Event

Product Reactions storing certain products, such as Styrene, that can polymerize with release of significant pressure. These tanks are often equipped with dome roofs and tank shells are anchored to a counterweight to prevent uplift.

External fire exposure is the basis for the NFPAminimum requirement.

Internal deflagration within an aboveground storage tank is the most severe but also a very rare incident. API research should produce a program to determine vent requirements based on product storage variables.

Internal deflagration within an aboveground storage tank is the most severe but also a very rare incident.

Earthquake is only a risk in certain areas but thedamage effects can be substantial. The sloshing ofproduct can cause uplift on the fixed roof and this maydamage or cause failure of the frangible joint, fixedroof support structure (cone roof columns, rafters, andaluminum dome supports and framing) and shell.

Shell and bottom damage resulting from seismic event. Note the plastic deformation due to uplift.(submitted by John Lieb, Tank Industry Consultants)

PRESSURE RELEASE ALTERNATIVES

Alternate Frangible Roof Joint Details for small tanks areoffered; however, cost to document these will likely prohibitadding these to the Standard.

Counterweight Anchorage of the tank shell and reinforcedshell to bottom joints.

PRESSURE RELEASE ALTERNATIVES (CON’T)

Relative Strength of Roof-to-Shell and Bottom-to Shell Jointsresearch completed will likely allow for further reduction ofthe 50’ diameter limit. Allowable uplift and impact on attachedpiping will also be considered.

PRESSURE RELEASE ALTERNATIVES (CON’T)

API 650 G “Structurally Supported Aluminum Dome” roofperipheral flashing (not sealed to tank) may be documentedto provide vent area contribution.

PRESSURE RELEASE ALTERNATIVES (CON’T)

API 650 G “Structurally Supported Aluminum Dome” roofperipheral flashing uplift (resulting in no shell damage) maybe repaired without removal of tank from service.

PRESSURE RELEASE ALTERNATIVES (CON’T)

Deflagration Venting research conducted with oversight fromthe PVRC and support from API provides a mechanism toevaluate venting of partial volume deflagrations.

CONCLUSIONTo minimize the impact of internal over pressure incidents, attention should be paid to the causes, frequency and resulting damage.

Some causes can be efficiently avoided.

Fixed and floating roof types and details allowing for internal pressures to be safely vented with minimal damage are preferred.

Attention must be made to tank operation, tank condition and modifications.

Determine vent requirements based on product storage variables and operating risks.