Failure in Boilers Economizer Tubes in Nitric Acid Plants

Sir this is supplier design data, i was also not able to believe how such defects can occur.And i am talking about Economizer which is in upstream of waste heat boiler, used in production of nitric acid

sir, i am talking about Economizer which is installed on upstream of waste heat boiler used in production of Nitric acid .

These are designed by well know design suppliers , is it design defect?

The pressure differential between the suction and discharge of each fluid stream is the main driving force of that stream. The pressure differential is affected by fluid flow rates, pipe surface friction, number of heat exchanger passes, bulk density and viscosity. Deposits, if present, reduce the available surface area and increase the pressure differential, thus resulting in inadequate flow. If a pressure difference is noticed, the system should undergo troubleshooting to identify the cause

Fixed tubesheet heat exchangers must not besubjected to thermal shock or premature failure may result. Thermal shock is a conditionwhere excessive stress is created in heat exchanger components due to differentialthermal expansion. Start the flow of both fluids gradually, introducing the cold fluidfirst. Reverse this sequence for shut-down; gradually stopping flow of both fluids,beginning with the hot fluid. (TEMA type L, M and N rear heads).

Tubes might be individually tested in order to provide a go/no-go test to determine whether the tube is failed or weakened and on the verge of failure. By treating each tube as an individual pressure vessel, the tube can be tested to pressure higher than the tubeside operating and design pressures, as per UG-27 of the ASME Boiler and Pressure Vessel Code, Section VIII, Division 1, which gives equations for calculating the maximum allowable working pressure (MAWP). UG -99 of the current Code prescribes the 1.3 multiplier times the MAWP for the allowable hydrotest pressure.

yes but here i am having shell side high pressure and tube side very low pressure.so want to know is it design defect or not. please check above mentioned data.

Thanks in advance

It not necessarily has to be a design error. When we have relevant differences between pressures in the exchanger design, it is best testing simultaneously shell side and tube side . This means filling the shell side and tube side simultaneously. Then go increasing pressure at the same time. This test is a manometer test and is frequent . The same rule applies for equipment start up.

wp

Were you pressurizing the tubes at 51 BAR? Or the shell at 51 BAR?

Did the tubes burst? Or were they crushed?

Is this a new installation that is being acceptance tested? Or one that has been in service for an extended period of time? (Normally initial acceptance testing should be performed once on new equipment and then all testing afterwards is executed using a decreased pressure value as-stated by the OEM for life of service.)

If you were pressurizing the tubes at 51 BAR the test pressure exceeds the tube design maximum pressure rating of 8.27 BAR by a factor of more than 6X.

The maximum tube pressure allowed for testing using the acceptance test 1.5 multiplier would be: 13.05 BAR

No device will withstand this type of abuse without experiencing some sort of failure.

If the tubes failed when the shell was pressurized at 51 BAR and this is a new installation I would suspect the tube wall is not adequate in thickness to withstand the applied pressure.

If the tubes have been in service for an extended time it could well be the wall thickness has degraded from oxidation to a value that will not function within design criteria.

If you want to solve the mystery, contact the tube manufacturer and discuss what happened with their field service representative.

I am certain that you need competent technical help at the site in order to identify what is wrong or what went wrong.

Finally! Common Sense has come out of her coma!

I am virtually certain the OP has pressure tested the tubing of this economizer at a pressure far beyond rating, but it could also be a crush failure. I give you a definite good answer.

Actually most of the other posters (not the OP) pointed out that pressure testing should be at equal pressure on tube and shell side for the high pressure test, and that the pressure be raised (and lowered) simultaneously (and slowly). It seems clear that the company OP works for failed to follow all the commissioning steps in correct order (if this was commissioning testing), and therefore any insurance on this should be null and void.

Oops is not the first utterance a young engineer should make.

I am pressurizing shell at 51bar during boiler inspection and shell side operating pressure is 34bar. during pressurizing tubes got burst.But from design data tubes design pressure is 8.24 and operating pressure is 7.5 bar.And plant is running normal since 5 years.

So want to know how in normal running plant condition tubes are not getting burst as external force of 34bar is applied on tubes external side, as shell side operating pressure is 34 bar and tubes are designed for 8.24 bar pressure only.

Here is a wild question for this plant.....

Was the economizer originally designed for another installation ?

Was the economizer "repurposed" without proper consideration of the new operating conditions ? This happens often in some parts of the world ?

No it's new commissioned plant and running under these condition normally from last 5 years.

Obviously, 34 bar less 7.5 bar is 26.5 bar external net pressure to the tubes under normal operating condition. This is a large pressure, thus the tubes must be fairly substantial. The tubes were probably designed (if designed for the specific plant being referred to) to handle perhaps up to 30 bar.

If now we bring the shell side up to 51 bar, and no data on what pressure the tube side was held to, then you could easily have exceeded the implosion strength loading of the tubes. Does that make sense to you now?

Based upon your last response to Mr. MJCronin, the problem must lie with the pressure testing protocol where the pressures between shell and tube sides are not maintained within the design envelope differential.

If, for example, the rated fail implosion strength of the tubes is 30 bar, and the shell side is to be brought up to 51 bar during test, then you MUST pressure the tube side also, up to the level of at least 21 bar, and probably should go further to be safe. The pressures have to maintained at a constant safe differential pressure of 18-20 bar, or the same implosion will take place again and again and again....

The explanation is simple: the pressure applied was greater than the tubes could withstand without deformation. However, that is the purpose of the test: to see whether all parts of the vessel can withstand the test pressure without damage. That the vessel failed the test is simply an indication, under controlled and relatively safe conditions, that the equipment is unlikely to withstand the operating pressure with an appropriate safety margin while in service on process fluids. Be grateful it failed during hydraulic test and not while in service!

As to whether or not it is a design problem, the equipment datasheets would give the requirements for the vessel in service. It is up to the assessor to make a judgement as to whether the materials selected under the appropriate test conditions stated in the datasheet, are likely to withstand the test pressure. One should always make an allowance for the condition of the equipment as it deteriorates over time, and previous test records of this and similar equipment will give some indication as to whether or not the equipment is likely to pass this time. If the assessor judges negative, then it behoves the Engineer to select replacement materials, replacement equipment, duplicate redundant equipment or hot/cold standby spares on the basis of experience and business needs. The individual in the best position to do this is the Original Poster.

As to whose fault it is, well, that is the responsibility of the Legal profession, which exists solely to make money out of other people's misery; such discussion would fall outside the parameters of what is acceptable on CR4.