Design and Maintenance of Refractory Linings in the ...

$: Design and Maintenance of Refractory Linings in the ...$
Design and Maintenance of Refractory Linings in the Reformer Section of

Ammonia Plants

M. Ballon, M. Havlik, K. Schmalstieg and M. Wolske

KARRENA GmbH

1. Introduction

ew technologies and the globalisation of the markets increase the competitiveness of all branches and industry companies. Increasing the

safety, reliability and efficiency are becoming more importance for successful organizations in the future. In this circumstance one of the substantial items of commercial plant units is the maintenance.

The operational availability and the most profitability of the production are associated with maintenance strategies. “Maintenance” has been developed to a real value-added factor for commercial plants as illustrated in figure 1.

The reported value of lost production is far higher than the direct maintenance costs (reproduced from [1]).

Figure 1: Results of survey of maintenance costs in 6 major companies

N

2006 165 AMMONIA TECHNICAL MANUAL

The area of conflict between reactive measures in case of emergency and proactive maintenance work is illustrated in Figure 2.

Figure 2: Optimal Mix of Proactive and Reactive Work

The targets for the maintenance of commercial plants must follow the guide line:

“Minimise the loss of production with moderate expenses for maintenance”

1.1 Maintenance of the refractory linings in the reformer section of Ammonia plants.

High temperatures and high pressures are typical for the reformer section of Ammonia plants. For this reason refractory linings are required in order to protect the steel vessels for the excessive process heat. The following figure 3 is showing the main refractory lined units of an Ammonia plant such as Primary and Secondary Reformer, Process Gas Cooler, Outlet Manifold System and Convection Bank. Each of these units has individual refractory linings in order to meet the specific requirements.

Figure 3: Ammonia Syngas Plant [©UHDE]

0% 20% 40% 60% 80% 100%

Level of Work Executed in Advance of Asset

Cost of Work

High

Low

Optimized Work

About 75-80% Proactive

2006AMMONIA TECHNICAL MANUAL 166

1.2 Control of the condition of the lining

The most effective control of the refractory linings has to be done during shut downs when inspections enable the evaluation of the conditions in front of the hot face skin. Such an opportunity is always to recommend at all units and at any shut down period although control of water consumption at water jacketing, development of hot areas at steel shell sections, fouling at heat exchanger tubes etc. can indicate irregular conditions of the linings even during operation.

Particularly the Hydrogen rich process gas can attack the refractory lining by Silica migration out of the insulation layers and could cause depositions at down stream units like the PG Cooler. At higher temperatures and higher pressures Hydrogen can remove Silica as per the following chemical reaction:

SiO2 (solid) + H2 (gas) SiO (gas) + H2O (gas)

In areas where lower temperatures are existing condensation of SiO takes place.

H2 + SiO H2O + Si

Fouling at heat exchanger tubes could be a typical result of this chemical reaction.

1.3 Preparation of Maintenance Activities

The documents of previous inspections, maintenance works and monitored operation data are to be used for identification of the required works and items to be prepared like:

• design improvements • material requirements and availability in

stock • specialists and labourers as well as

equipment and tools etc.

2. Primary Reformer

Synthesis gas necessary for production of Ammonia will be generated by using the steam-reforming process in a Primary and Secondary Reformer. The synthesis gas will be lead through the catalyst tubes inside the Primary Reformer box. The max. hot gas temperatures are in the range of 1200 °C.

2.1 Primary Reformer walls

The walls are typically lined with insulating products. Most of the designs are based on using either insulating firebricks or ceramic fibre materials.

2.1.1 Insulating Fire Bricks

The lining system above the tunnel walls is built up of insulating fire bricks and block insulation for the back-up layers. The side wall lining is supported by brackets at different elevations. At these bracket areas expansion joints are filled with ceramic fibre materials. During maintenance these expansion joints are to be cleaned and refilled with ceramic fibres. The condition of the hot face brick layer should be controlled with regard to cracks


Figure 4: Insulating fire brick lining

Figure 5: Bracket Area Figure 6: Brick Lining

2.1.2 Ceramic Fibre Materials

Ceramic fibre walls at Primary Reformers could be either designed with blankets or modules. The ceramic fibre blanket lining is consisting of different layers fixed by fibrefix pins and clips/cuplocks. The number of layers is dependent on the operating temperature and heat flow through the wall.

Figure 7: Ceramic Fibre blanket lining


Picture 8: New CF blanket lining During a shutdown the fixings arrangement are to be checked. The blanket layers should be in good shape. In case parts of the fibre blankets are affected, the blankets can be replaced. During these works the conditions of the fixings are to be checked. Cuplocks and washers might be replaced as well.

Picture 9: Ceramic fibre module lining of a transition section after 4 years operation

Picture 10: Roof Ceramic Fibre module lining

Ceramic fibre modules are fixed at the steel shell by using different fixing systems.

During shutdown such a lining is to be checked with regard to the size of joints between each module. Bigger joints of let say more than 10 mm width must be filled with ceramic fibre

blankets using a special method anyway. Smaller joints will be closed again during operation because of thermal expansion.

Figure 11: Method of filling joints in between modules

2.2 Primary Reformer Floor

The typical lining starts from hot face with special shaped high duty firebricks in the tube passages and ends with insulating layers to lower the temperatures of the bottom steel.

Figure 12: Overview Floor


Figure 13: Details Tube penetration

During shutdown cleaning of the floor and replacement of the expansion joint filling is required

2.3 Tunnels

Particular attention must be given to the design of the flue gas tunnels with regard to the pressure difference between outside and inside of the tunnels. The typical design of the tunnels is considering high duty fire clay bricks for the

walls, cover slabs are made of similar grade of material. During maintenance all expansion joints are to be cleaned and filled with ceramic fibre blanket in order to enable proper expansion during the next run and to seal the tunnel walls.

Picture 14: Expansion Joint and Tunnel cover slabs after 5 year of operation

Furthermore the cover slabs must be checked for cracks and spalling, and needs to be documented and observed for the next shutdown. In case of serious damages the blocks need to be renewed.

Figure 15: Tunnel Walls

High duty Insulation bricks

Block Insulation or castable

High duty firebricks

Insulation bricks

Block insulation or castable

Ceramic fibre material

Expansion Joint


Picture 16: Damaged cover slabs (after 10 years of operation period)

2.4 Roof

Most of the current roof designs are based on ceramic fibre linings - again - either designed as blanket or module lining. The required

maintenance of the roof lining is similar to the ceramic fibre walls. In case of using ceramic fibre blankets for the roof lining special attention has to be paid to the fixing arrangement by cuplocks. Because of high temperatures and sometimes due to vibration of the burners the cuplocks tend to getting loose. Therefore at least the visual inspection is important and re-fixing of the cuplocks as required.

3 Outlet Header System / Transfer Line

The process gas of the tubes of the Primary Reformer is collected in the outlet header system. The refractory lining system of the header system is consisting of insulating castable for the back-up lining covered by a metallic liner or a dense castable as illustrated in figure 17.

Figure 17: Two different lined Header systems


Normally no maintenance activities are required. Renewal of seals on header/transfer blind flanges regularly to be opened for inspection is required. During changing of the reformer tubes all “donut-bricks” (special shaped bricks) should be checked and replaced if required.

4 Secondary Reformer

The process gas leaves the primary reformer and will be fed into the Secondary Reformer. The feeding temperature of the process gas is in the range of 800 - 850°C.

The gas temperature will be increased at the Secondary Reformer combustion section to approx. 1.300°C whilst passing the catalyst the gas temperature will be reduced by the

endothermic process to approx. 1.000 °C. The operating pressure of modern units is around 40 bars. Due to the catalyst in between gas inlet and outlet of the Secondary Reformer a pressure drop which is limited to a certain level has to be compensated by the catalyst vault which has also to carry the weight of the catalyst.

It is extremely important that the refractory design does not enable process gas bypassing through the lining. For this reason the hot face layer material has to have low gas permeability and the expansion joints are to be closed during operation of the unit at higher temperatures.

Figure 18: Secondary Reformer of different designs

Synthesis gas flo

High Alumina Refractory Bricks

Bubble Alumina shapes

Bubble Alumina Insulating Layer

High Alumina Castable


It is important to know that the vertical and horizontal expansion joints are determined in accordance with the design temperature, i. e. during cold shutdown the joints must be opened and during operation closed again. Operating temperatures above design would cause additional forces induced by the non free expansion.

The vertical and horizontal expansion joints must be checked and maintained during each shut down period.

The joints are to be cleaned, grains or dust should be removed. The condition of the bricks around the joints has to be checked with regard to the edges, quantity of cracks and spalling.

The vertical and horizontal expansion joints should be checked and maintained during each shut down period.

The joints are to be cleaned, grains or dust should be removed. The condition of the bricks around the joints has to be checked with regard to the edges, quantity of cracks and spalling.

The radial expansion joints are required at all sections of the Secondary Reformer lining in order to enable free thermal expansion of the lining.

The total joint thickness, i.e. the cumulated figure of all joints around the circumference of the brick ring is to be checked in accordance with the design documents. Cleaning of the joints is very important to avoid any damages as a result of non free thermal expansion of the lining.

Figure 19: Typical horizontal joint Figure 20: Typical radial expansion joints

Expansion Joint, check and keep clean


Picture 21: Detail of the expansion joint after approx 10 years operation time

Picture 22: Typical cracks of the lining after approx 10 years operation time Rounded brick edges and glazy hot surfaces are caused either by very high operation temperatures (glasification) or in general with the advanced operation time period.

Risk analysis for any further operation requires experienced specialists.

5 PG-Cooler

The synthesis gas of the Secondary Reformer will be introduced to the inlet chamber of the PG-Cooler (Process Gas Cooler). The gas temperature is in the range of 1000 °C and the gas pressure is around 40 bar. The gas will be cooled down by passing through the heat exchanger tubes to the outlet chamber. The PG-Coolers are typically lined with a dense material for the hot face layer and an insulating back-up layer.

Due to the process conditions and the turbulent gas flow, the integrity of the refractory lining is

very important. Special attention has to be paid to the transition area of the tube sheet lining and cylindrical brick lining as well as the lining of the conical section. Maintenance activities are limited to checking of the expansion joints, as well as cleaning and re-filling - if required- during each shutdown period.

Figure 23: Detail of PG-Cooler Lining (Inlet Chamber

Figure 24: Detail of PG-Cooler Lining (Outlet Chamber)

6 Moderate Operation

Immoderate operation can cause extreme damages to the refractory linings of the different plant units. Some items are to be considered relevant in this circumstance like:

High Duty Refractory Material

Insulation Material

From Secondary Reformer

Heat Exchanger

High Alumina Refractory Insulation

Material


- operating temperature - dry-out and initial heating-up - heating-up and cooling down cycles and

rates - pressurisation and depressurisation rates

Indications about the condition of the refractory system can be kept by analysing operating data like the temperature of pressure shells, higher water consumption in case of water jacket, fouling at the heat exchanger tubes caused by Silica removal from the refractory insulation layers etc.

6.1 Operating temperature

For the integrity of the lining it is always important to consider the design conditions of the plant unit. The selection of the most suitable refractory system and the design is precisely based on these conditions.

One of the most important properties of refractory products is the classification temperature of the materials. The hot face temperature plus an addition of 100-150°C will determine the classification temperature of the refractory materials.

6.2 Dry-out and initial heating-up

Dry-out and initial heating-up of the refractory lining is an extremely important process which requires consideration of particular items. Water required for working with unshaped refractory materials will be kept in the set bounding matrix as physically adsorbed water and chemically bounded as part of the hydraulic bounding system. The physically bounded water has to be removed during dry-out at temperatures in the range of 100-200°C whilst the chemically bounded water will be removed at higher temperature above 450°C during initial heating-up.

DURATION [h]

HO

T G

AS T

EM

PE

RA

TUR

ES

[°C

]

max. 1

0 °C/

h

130

x = without regulation

20

100

X

hold for 48 h

300

200

ΔT =

1 0 K

400

ΔT =

1 0 K

curve nottrue to scale

max. 30 °C

/h

Figure 25: Typical dry-out curve

DURATION [h]

HO

T G

AS

TEM

PE

RAT

UR

ES

[°C

]

20

100

300

200

400

500

service temperature

curve nottrue to scale

max. 20 °C

/h

130

x = without regulation

600

max

. 50

°C/h

700

800X

X

max. 30 °C/h

Figure 26: Typical heating-up curve

Dry-out of the refractory lining is the advanced process of initial heating-up for most of the plants, particularly important in case the initial heating-up requires increasing of the inside pressure of the units.

Always bear in mind that the refractory lining in a common size of new installed Secondary Reformer contains approx. 20-25000 litre water. In case the unit will start operation without prior dry-out the water will be not removed because of the high pressure inside. This can be verified by considering the phase diagram for water. This diagram is clearly showing that the water will be held in the unchanged modification when the unit operates at high pressures. In the unlikely event that the inside pressure during operation will be uncontrolled and suddenly decreased the water will immediately be vaporised by an extreme volume increase, an effect similar to an explosion.


Figure 27: Phase diagram for water

6.3 Heating-up and cooling down

During heating-up it is important to avoid too high thermal gradients at the layers.

The different thermal expansion of the hot and back sides of each layer could cause stresses in the refractory matrix which could result in spalling and cracking. Tensile stresses are effectively damaging the back side of the layer during too fast heating-up.

∆ L

∆ L

Stresses inside the layer during heating-up

T hotface > T backside

T hotface T backside

↑

↓

tens

ile s

tress

es

Figure 28: Stresses inside the layer during heating-up

During cooling down the effects inside the layer are basically the same but vice versa. Too fast cooling down will induce tensile stresses at the hot face skin of the layers due to the different temperatures inside the refractory matrix.

6.4 Pressurisation and depressurisation

Pressurisation and depressurisation always need to be done very carefully because of the nature of refractory products with the inelastic material structures and the less gas permeability of the hot face layer material typically installed in Secondary Reformers and PGC inlet chambers.

Refractory linings with more than one layer will exceed pressure equalisation as a function of time in case of non open gaps and joints. The time required for pressure equalisation is mostly a function of the gas permeability of the dense refractory layer, only. In case of a too fast depressurisation the gas trapped in the back-up layers cannot escape fast enough due to the more or less dense hot face layer. This can cause considerable damages to the lining system. KARRENA has developed an own computerised program for investigations on pressurisation and depressurisation of refractory linings of synthesis gas units. The current state of the results with regard to proper pressurisation and depressurisation rates is shown in figure 29.

5 m

in5

min

5 m

in

5 m

in

0,5

bar/m

in

0,5 bar/min

ΔP = 2,5 bar

ΔP = 2,5 bar

ΔP = 2,5 bar

ΔP = 2,5 bar

Figure 29: Typical pressurisation and de-pressurisation rates

The quantity of gas passing through the refractory system depends on the gas permeability of the dense layer with the lowest amount of open pores. This is the layer relevant for specifying pressurising and depressurising rates.


7. Documentation

Proper visual inspection, measurement of joints and photo documentation during each shutdown will enable a proper evaluation of the lining condition and will clearly indicate required activities in the future. Such well prepared maintenance documents are forming the integral basis for works to be planned during the next shutdowns.

8. Appendix

[1] Leonie Gouws, Adrian Stephan and James Trevelyan: The nature of engineering maintenance work: a blank space on the map, School of mechanical engineering, The University of Western Australia 2006


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