5 replies to this topic

[Ade24]

Hello everyone, 

I have a topic regarding the blowdown system, 

In this system, the blowdown KO Drum discharge to atmosphere and not to flare. I have considered a nitrogen purging in order to prevent oxygen ingress and the possibility of detonation within vent system.

 

The team is divided, however, as other process engineers suggest avoiding the use of nitrogen and designing the whole system to a design pressure of 40 barg. 

They argue that a pipe designed for a static load of 40 bar can withstand a much higher dynamic pressure.

This pressure seems very high to me and I would like to have a reference before designing a system with such a high pressure.

Also, as far as I know, a design pressure of 7 barg should be considered for systems of this type if no notrogen is available.

Does anyone have a reference on this?

 

Regards

 

 

 

 

[shvet1]

Your colleagues can design the pipe that is able to withstand an internal detonation but such will require a science team from the rocketry or military industry and I am afraid of the total cost of the final pipe.

 

Reply them that 40bar is related to the static stress that is being applied from 0 to 40 bar during hours. While the internal deflagration explosion is related to dinamic stress that is being applied during a second and the deflagration explosion compensation method as per the industry accepted practice is limited to the design pressure 3.5 barg, not 40 barg.

The internal detonation explosion or detonation transition explosion is so powerfull and rapid that is related to transient conditions and is not covered by any industry stress code and therefore is irrelevant to metal design stress or pressure.

 

This effect is relevant to explosions and pseudo-explosions when the stress is applied during miliseconds and a pressure front/wave propagates at velocities near the sound velocity.

 

The common explanation of transient conditions, that you may find in relevant industry literature, is a beer wooden crate on a pub yard. One can apply a significant weight on top of it much higher than it is originally designed for provided that it is applied slowly and carefully. E.g. a pub visitor can step and stay on top of it. In the opposit of that the crate will be readily flattened when the same person jumps on it. 

The same is valid for pressurized pipes - there is a huge difference whether the stress is able to dissipate evenly through the metal or applied locally.

Edited by shvet1, 29 November 2024 - 07:36 AM.

[Pilesar]

Consider a detonation arrestor or flame arrestor for the vent. Consider a water seal.

[shvet1]

any potential flow restriction at blowdown path is dangerous and requires design pressure should of an upstream pipe  be equal to that of a blowdown source

liquid seal can or can't prevent detonation depending on flame backflow path length

 

as always details are crucial

 

see chap. 12 for references

https://a.co/d/7Yo99E8

Edited by shvet1, 29 November 2024 - 09:27 AM.

[Ade24]

Thank you for your reply.

 

So are we saying I can only purge with nitrogen? what happens if nitrogen is not available?

[shvet1]

Choosing the purge gas between N2, methane, fuel gas is subject to ecnomic&safety studies both, e.g. see BP's pratice (most other Majors use the similar approach)

 

 

BP's std. GP 44-80

 

11.5. Purge gas for flare and atmospheric vent systems

Purge gas systems installed on flare and atmospheric vent systems shall comply with the following general requirements:

a. A continuous purge system shall be used for closed flare and atmospheric vent systems to prevent air ingress and a potential explosion hazard.

It is not necessary to purge individual atmospheric relief devices. 

b. In special cases, atmospheric vent systems may use a robust design that is capable of withstanding an internal explosion (detonation). Use of this approach shall be subject to entity EA approval in consultation with fire and blast experts.

It is possible to build a vent system that is sufficiently strong enough to withstand an internal explosion inside the vent system. This is not done in flare systems due to their size and complexity in most processes. However, explosion rated vent systems are occasionally used to eliminate purge gas. The explosion rated design option is best suited for small well pads or NUI offshore platforms, where relatively few items of equipment and a simple process is connected to the vent system. 

Internal vent header pressures created by explosion overpressure can be as high as 5 500 kPag to 6 900 kPag (800 psig to 1 000 psig) in the vent system and shall be considered in the design for explosion rated vent systems. This overpressure was calculated based on an 8 in vent pipe approximately 6 m (20 ft) long. 

c. The purge gas shall be an inert gas (e.g., nitrogen), fuel gas, or natural gas from a reliable source. The choice of purge gas should be evaluated on the basis of lifecycle cost, environmental impact, and safety issues and the following:

1. Current cost basis of greenhouse gas emissions shall be considered in selecting the purge gas source, and lifecycle economics should be factored into the overall decision.

2. Use of site generated nitrogen in this application may require evaluating the potential for oxygen entry into inert gas stream from the nitrogen generator.

In choosing between fuel gas and inert gas purge, it should be noted that 1 volume of fuel gas produces approximately 10 volumes of inert gas in an inert gas generator. However, the inert gas generator would be expected to have a backup supply provided, since the generator is inherently less reliable than its feed gas supply.

3. Automatic backup supplies should be used, if necessary, to achieve an acceptable overall reliability.

4. Purge system shall be designed such that loss of a single purge gas source or injection point does not cause hazardous conditions to occur.

d. A minimum purge gas velocity or flowrate shall be maintained at the atmospheric vent stack tip or flare tip to:

1. Minimise air ingress due to wind or buoyancy effects.

2. Prevent burnback inside the flare tip (for flare systems).

e. The larger of the two flowrates in d., as appropriate, shall be used.

If an inert gas is used, the flashback velocity (the speed with which a flame travels through the mixture) is significantly reduced. This has the advantage of significantly reducing the risk of a damaging explosion in the event of an unforeseen occurrence, such as a suckback.

f. The minimum purge rate to minimise air ingress into a flare or vent system due to wind effects shall be calculated using the Husa correlation formula shown in ISO 23251/API Std 521, “Design of disposal system components” clause and “Flares” subclause, as follows:

1. Purge rate shall be such that the oxygen content in the disposal stack gases 8 m (25 ft) down from the discharge point shall be less than 6%. 

2. The Husa purging correlation should only be used if design wind speed does not exceed 13,4 m/s (30 mph). Locations where prevailing winds often exceed this wind speed may need higher purge rates.

g. If either the purge gas or discharge process gas is low molecular weight (e.g., containing high concentrations of hydrogen), the maximum oxygen content in 

f. shall not exceed the following:

1. Maximum oxygen of 5% vol for flare gases with MW greater than 6 but not exceeding 8.

2. Maximum oxygen of 4% vol for flare gases with MW greater than 4 but not exceeding 6.

3. Maximum oxygen of 3% vol for flare gases with MW less than 4.

h. To verify that purge rates maintain safe oxygen levels as specified in f. and g., the flare or vent stack should have an oxygen monitoring system in accordance with GP 22-20.

The reliability of these O2 monitoring systems should be considered before installation, as well as the potential use of portable O2 analysers.

i. When recommissioning an air filled flare or atmospheric vent network, including the header and associated piping, liquid knockout drum, and stack, a supplemental, high initial purge flow may be required to free the entire disposal system of air.

j. Every disposal system shall have purge gas connections at header extremities.

These connections are required to ensure that the system is air free before introduction of hydrocarbon.

k. Continuous purge gas used during normal operation should be installed near the knockout drum and/or at header extremities.

Generally, continuous purge gas is introduced near the knockout drum, as it is closest to the primary source of air ingress (i.e., flare tip or vent discharge point).

l. Minimum instrumentation for purge gas systems should include:

1. Flow indication and control.

Generally, a rotameter and globe valve installed on the supply are used for flow indication and flow control.

2. Low flow alarm. 

Edited by shvet1, 02 December 2024 - 04:14 AM.