Mr. Hrishkesan,
In addition to the nice explanations of Mr. Jignesh Rana, here're some more insights on this issue.
High temperature hydrogen attack is a metal degradation phenomenon that is well known in the petro chemical/refining industry. It occurs in carbon and low steels exposed to high partial pressure of hydrogen at elevated temperatures. Such damage can occur over time and reduce the fracture toughness of the material. The damage is caused by the seepage of hydrogen that reacts with metal carbides to form methane gas. The source of hydrogen is the hydrocarbons in the flow stream. This reaction decarburizes the steel, produces micro cracks and lowers the toughness of steel without necessarily producing a loss of thickness.
MC+4H=M+CH
4 ----- Equation for HTHA.There're empirical equations to quantify such damages.
One such approach is to use an empirical parameter, called Pv. In addition to temperature and hydrogen partial pressure, the Pv parameter includes the exposure time. The exposure time accounts for the incubation time. Pv. The PV parameters reflected in API 581, as stated above:- 
where T = operating temperature (in kelvin), age is the in-service time, hours, PH2 = hydrogen partial pressure (in kilograms per square centimeter). In this case you need to consider T= 700 DegC, due to the exposure of the steel.
As a matter of precaution in addition to the replication suggested below UT inspection , either AUBT or TOFD (see the attachment) would be advisable. Such inspection may be a part of the periodic maintenance inspection program as required.
A short time but sudden exposure may not reveal any significant damage.However the temperature is sufficiently high for any mobility of hydrogen in the steel. Hence there may be some damage manifestations later. The above suggested NDE in addition to replication would help to safeguard your equipment.
Thanks.
Pradip Goswami,P.Eng.IWE
Welding & Metallurgical Specialist
From: materials-welding@googlegroups.com [mailto:materials-welding@googlegroups.com] On Behalf Of jignesh.rana@Linde-LE.com
Sent: Thursday, June 21, 2012 1:04 AM
To: materials-welding@googlegroups.com
Subject: Re: [MW:14627] CS reactor exposed to temperature run away in presence of H2
Dear Mr. Hrishkesan,
From the provided data and background information, the concern is about HTHA. (High Temperature Hydrogen Attack) . Request to provide the important parameters - operating conditions and partial pressure of Hydrogen, in order to access the detailed effect to HTHA.
Request to refer API 941, which is widely accepted standard to defines safe limit of HTHA inspection methodology and factors influencing HTHA. I am referring certain extract from API 941, for your case.
Nelson curves defines the safe operating conditions for long term exposure to hydrogen service. First of all, determine safe operating conditions for your reactor.
Please note that HTHA begins once service conditions ( temp, partial pr. of hydrogen) allows diffusion of hydrogen is steel. HTHA is time bound attack, during initial phase the deterioration is so microscopic that can't be detected with current NDE and metallographic examination. After this period, there is time, when no noticeable changes in property, that could be determined by current examination method. After this time period, there will be noticeable change in properties of material, which may be detected by NDE / metallographic examination.
This phenomenon varies with type of steel, severity of exposure, and other factors. It may last only few hours under extreme conditions may become progressive at lower temp, and partial pressure of hydrogen.
The length of the incubation period is important, as it defines the safe conditions of the equipment. The incubation curve for carbon steel is given in Fig. 3 of API 941.
Pl find the safe conditions with short term exposure (450 Deg C for couple of hours with partial pressure of H2) . If co incident point is below the curve, still no noticeable effect of HTHA.
But, the short term exposure at 700 Deg C for 15-20 minutes is quite severe and may become fatal. Pl take replica and check micro structure for decarburization and fissuring.
Pl refer Ann. E of API 941, for summary of Inspection methods for HTHA and access the situation.
With Best Regards,
Jignesh R Rana
Manager - Materials Technology, QA/QC & Standards
Linde Engineering India Pvt. Ltd.
'Linde House'
Opp: VUDA Office, VIP Road, Karelibaug,Vadodara - 390018, India.
Tel.: +91 265 3056789, Fax: +91 265 2335213 , Mobile: +91 90999 86019
jignesh.rana@linde-le.com, www.linde-india.com
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sudharsan s ---21/06/2012 09:02:17---What was the pressure during the excursion of high temperature? What was the partial pressure of Hyd
From: sudharsan s <sudharsan.inspection@gmail.com>
To: materials-welding@googlegroups.com
Date: 21/06/2012 09:02
Subject: Re: [MW:14621] CS reactor exposed to temperature run away in presence of H2
Sent by: materials-welding@googlegroups.com
What was the pressure during the excursion of high temperature?
What was the partial pressure of Hydrogen during sweeping?
For High temperature Hydrogen attack to occur, it requires some incubation period. Even then, look from that perspective also
On Tue, Jun 19, 2012 at 5:23 PM, K.M Hrishikesan <hrishikm07@gmail.com> wrote:
Vessel Description: Reactor
MOC: SA 516 gr 70
Design temperature: 200 deg C
Design pressure: 44.6 kg/cm.sq
Thickness: 36 mm for shell; 50 mm for the dished end
Service: Hydrogen environment in the presence of catalyst
Above equipment was accidentally subject to temperature run away during Hydrogen sweeping. As a result the temperature shot up abruptly to the range of
700 deg.C. for 15-20 minutes and 450 deg c for approximately a couple of hours.We have checked the hardness which is found to be OK. In-situ metallography is underway.
My queries to metallurgical experts are
- 1. What will be the metallurgical impact on integrity of the vessel due to above incident?
2. How can we assess the damage/degradation which might have occurred on the of the vessel in terms of its metallurgy? please suggest some NDE's
Regards
Hrishi
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With Regards,
Sudharsan
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