Rapid Autoclave Blowdown 04.15doc

© Copyright 2015 MOGAS Industries, Inc.
14330 East Hardy Street Houston, TX, USA 77039-1405 www.mogas.com
ph +1.281.449.0291 fax +1.281.590.3412 email mogas@mogas.com
CONTROLLED RAPID AUTOCLAVE BLOWDOWN
By
M.Pearson, L.Knightingale-Mercer, K.Jackson & Jansen Scheepers
Hatch, Canada
Mogas Industries, USA
Presented by
Kevin Jackson
kjackson@mogas.com
First Presented 2009
Updated Operational Conclusion April 2015
CONTENTS
1. BACKGROUND ......................................................................................................
2. APPLIED TECHNOLOGY.......................................................................................
3. SIZING DATA .........................................................................................................
4. CONCLUSION ........................................................................................................

(a)
(b)
Fig 1
(a)
Quench
Vessel
Autoclave
Vent Line
Background
Traditionally brick lined autoclaves used in the High Pressure Acid Leach (HPAL) and Pressure
Oxidation (POX) leaching process of minerals such as Nickel, Gold and Copper, have a requirement
to be depressurized or “blowndown” due to process upsets, emergency situations, allow access for
essential maintenance, equipment replacement or failure.
The autoclave is fitted with a vent line that maintains the
pressure in the autoclave. The vent line should not be confused
with the “discharge” line that maintains the liquid level in the
autoclave.
The vent line consists of isolation valves (fig 1 a) and a control
valve (fig 1 b). Typical vent line sizes are 4” - 8” ANSI 600 class
that contain two 8” high integrity severe service metal seated
isolation ball valves that hold pressure in the autoclave when
required and a 4” or 6” angle pattern globe control valve with
actuator and 4/20mA control signal that controls the release of
the elevated gas pressure from the autoclave into the quench
vessel. This maintains desired pressure and prevents over
pressurization of the autoclave.
Autoclave blowdown takes place using the vent line. The
blowdown rate is governed by two factors 1) the brick lining is
subject to thermal shock if the temperature inside the autoclave
changes too quickly and 2) the capacity of the control valve.
Generally the limiting factor for autoclave blowdown has not
been the thermal shock of the brick lining as you would expect,
but, the capacity of the globe control valve.
The initial design pressure in the POX autoclave is
approximately 3600 kPag at 180
o
C – 240
o
C and HPAL operates
at approximately 6000 kPag at 240
o
C – 270
o
C and pressure in the quench vessel is atmosphere, so
the capacity of the control valve to handle flowrates at this ∆P needs to be small, around 25 to 35 Cv.
When the autoclave needs to be blowndown the angle
pattern control valve releases pressure in the autoclave as
it does in normal operating conditions, however as the
pressure in the autoclave reduces the ∆P (difference in
pressure in the autoclave and the quench vessel) the
capacity required to pass the flowrate increases beyond
the capacity of the angle pattern globe valve and therefore
takes longer to depressurize the autoclave completely.
Globe control valves by design have limited rangeability
(the ratio of the maximum controllable flow to the minimum
controllable flow), this is because of their linear
construction. Their ability to handle large scale differences
in flow and/or prerssure parameters inhibits their use in
blowdown and other applications that require rangability
above 40:1
The rangeability for the rapid autoclave blowdown is
approximately 70:1
Typical angle pattern valve layout

Applied Technology
Within the oil & gas industry it has long been the practice to let pressure down across a multiple
number of stages. There are a number of reasons for this, but primarily it is done when controlling gas
flow to reduce velocity, noise, erosion and vibration. Of course this multi-stage letdown or torturous
path technology has been applied in the past to relatively clean gases.
The principle operation of the torturous path for gas flow is
to control the pressure drop at each right angle turn, thus
limiting the velocity and noise. With less velocity there is less
erosion and vibration.
The number of right angel turns in the FlexStream® trim can
vary from 2 to 36, the number is dependent on the ∆P
across the valve and flowrate.
The autoclave blowdown gas has some solid particle
carryover, so by controlling the velocity the FlexStream®
trim is limiting the speed at which the particles collide with
the trim and therefore limiting the amount of potential
erosion. It is crucial that velocity in the flow passages is not reduced to a level where solids can stall
in the trim because they don’t have sufficient speed to pass through the path.
It is important that the size of the path is sufficient to allow the volume of particles to pass through the
trim. The solids content for this particular application is estimated at a maximum of 200kg/hr or <2%
by volume. Any particle that is too large to travel through the trim will gather at the bottom of the ball
and be discharged when the valve is in its fully open position.
Because of the manufacturing techniques used in the production of the FlexStream® trim path, sizes
can be changed by simply changing the thickness of the plates that make up the trim element.
Rangeability is the key issue as stated before, any additional control valve added into the piping
arrangement must be capable of handling the changing pressure drop and flow rates associated with
the depressurization of the autoclave. The unique design benefit of FlexStream® is to have
rangeability in excess of 300:1
Unlike a linear globe valve the FlexStream® trim is not
linear and therefore not held by a bonnet assembly
arrangement that compresses the trim and/or seat in
place. The trim or seat is not screwed but is
independently held within the ball by a removable
retention ring. This means that the ball ID can be filled
from 10% to 100% to ensure that the right amount of
trim can be inserted to deal with the high ∆P and
flowrate where required. As ∆P reduces the ball is
opened up producing the hydraulic diameter that allows
low ∆P and high flow rates.
Transition across the flow cases may require less
pressure letdown turns due to the reduced ∆P, this can
be accommodated within the FlexStream® trim element
but by changing the number of pressure letdown turns
across each torturous path, in some cases just down to
the drilled exit holes.

Applied Technology
The advantage with the trim element being in the ball is that the surrounding valve body, stem, seals,
gaskets and actuators are interchangeable with the existing isolation valves, this means less training
for site or workshop based maintenance staff.
Therefore there is no introduction or concern surrounding the viability of the base valve design as it is
already proven in use.
The revised vent line system layout would include for additional 8” pipe run connecting the new rapid
blowdown line to the quench vessel, the FlexStream® control valve, actuator, 4/20mA positioner and
additional I/O counts in the PLC/DCS.
The additional FlexStream® rotary control valve would be
fitted with an actuator capable of accepting a 4/20mA
control signal for precise position control, this actuation
can be achieved with pneumatic, hydraulic or electric
power sources.
The normal mode of operation would be exactly the same
as before, the control valve (c) would be closed and
control valve (b) would be regulating pressure in the
autoclave. If the autoclave was called upon to
depressurize, the control valve (c) would position itself
using the control loop and based upon temperature the
valve would position its self to allow gas and pressure to
pass into the quench vessel, this process would continue
until the autoclave had reached atmospheric pressure and
the normal safety procedures for entry could be made.
This system is best incorporated in initial plant design and
layout, but can also be retrofitted.
(a)
(b)
Fig 2
(a)
Quench
Vessel
Autoclave
Vent Line
(c)

Sizing
The FlexStream® sizing that has been carried out in accordance with ISA S75-01. The chart above
shows the 5 basic operating conditions of the valve that has a maximum pressure drop ratio of 1
( calculated as XT = ΔP / P1 ) . At each case the flowrate, ∆P, Cv, noise, kinetic energy and velocity is
calculated to ensure they are not above the limits allowed by the standard or best practices for the
industry itself.
The sizing shows that the Max and Norm cases have a Cv of 30 or less at a flow rate of between
3,573 and 1,989 kg/s with declining inlet and outlet pressures. These flow cases are passing through
the 8 turn torturous paths with velocity and noise control taking place due to the significant ∆P present.
This can be seen on the Cv – Stroke graph below
The inlet pressure reduces as the
pressure in the autoclave decreases and
the outlet pressure to the quench system
also decreases. The flowrate decreases
but has less impact on Cv as the ratio of
inlet to outlet pressure remains steady.
As the pressure in the autoclave
decreases the gas volume increases so it
takes longer to release the gas through
the restricted orifice in the angle control
valve. With the high rangeability in the
additional valve it simply opens furth to
allow the higher Cv to pass the increased
gas volume.
At the Min case the ∆P is 2kPag and
requires a Cv of 1303 to be able to pass
the flowrate of 1,989 kg/s, in this position, i.e. with the valve fully open some of the flow will pass
through the torturous paths but the majority will flow down the hydraulic diameter.
P
R
O
C
E
S
S
.
D
A
T
A
FLUID Autoclave Gas/Vapor
UNITS Max Norm Norm Norm Min
FLOW RATE kg/s 3.573 3.573 2.871 1.989 1.989
INLET PRESSURE KPa(g) 3618 3352 2352 1352 100
OUTLET PRESSURE KPa(g) 0 2352 1352 100 98
INLET TEMPERATURE Deg. C 230 230 165 100 100
MOL. WEIGHT 32 32 32 32 32
RATIO OF SPECIFIC HEAT 1.54 1.54 1.42 1.42 1.3
COMPRESSIBILITY 0.850 0.850 0.925 1.000 1.000
INLET DENSITY lbm/ft3 2.091 1.941 1.455 0.936 0.130
KINETIC ENERGY Psi 61.00 3.44 3.58 8.47 0.31
CALCULATED CV 20 30 29 27 1303
STEM TRAVEL % 32.1% 42.6% 42.0% 40.2% 98.6%
SPL AT 1 METER dBA 89.4 75.0 75.0 75.0 75.0
OUTLET PIPE VELOCITY Mach 0.144 0.011 0.015 0.060 0.084
VALVE OUTLET VELOCITY Mach 0.148 0.011 0.015 0.061 0.087
Cv - Trim / Open / Turns 30 1302 8 30 1302 8 30 1302 8 30 1302 8 30 1302 8

Conclusion
By using the controlled rapid autoclave blowdown rotary control valve, blowdown times for the
autoclave can be reduced from 24+ hours to approximately 13 hours, this represents a significant
reduction in turnaround times for the autoclave and can have dramatic impacts on run times and costs.
Irrespective of the process, HPAL or POX, the impact of this reduced depressurization time is
reflected in increased production and will always be significant to the plants efficiency. In the case of
gold (our example) this is magnified due to the base metal price.
Operational Conclusion – April 2015
The implementation of the MOGAS
FlexStream® “Rapid Blowdown Valve” – RBV
(Million Dollar Valve) does not interfere with
the day to day mass balance operation of the
plant – appendix PROCESS BLOCK FLOW -
Typical Pressure Oxidation.
The MOGAS FlexStream® RBV is installed to
dramatically reduce the blowdown time of the
autoclave and is only used when a decision is
made to take the autoclave offline for
maintenance of equipment.
The objectives at the outset were for MOGAS
to:
1. Have interchangeability between the
same size valve bodies used in
isolation.
2. Achieve a 50% reduction in
conventional blowdown times.
3. Have the same operational longevity
as the proven MOGAS isolation valve.
The MOGAS FlexStream® design was
based around the 8” 600# C Series metal
seated “mining” isolation valve fitted with a 5
step response 4/20mA self-contained
electro-hydraulic actuator. The only
difference between the C Series Isolation and FlexStream®
Control valve is the downstream seat.

The MOGAS FlexStream® rotary control valve was installed during the main plant construction and
became operational during commissioning, start up and full production during late 2012 early 2013.
MOGAS has been working closely with the operator full filing the MOGAS MORE service support
contract and has been able to monitor this application and performance through the actuator
diagnostics for the last 2 plus years.
Naturally the FlexStream® RBV remains closed during
routine operation of the autoclave. The valve has recently
been returned to MOGAS for service after recording 5
operations during that 2 year period – an operation being
from closed - open - closed.
Data from site showed that the FlexStream® RBV blew
down the autoclave to a safe working level in 13 hours.
The trim has shown some signs of product build up and
wear, but it is important to say that the valve did not fail in
service. The decision was taken remove the valve for a full
performance evaluation to enable new performance criteria
to be set for the future.
The return on investment is a relatively simple calculation :
Return On Investment (ROI) = (Cost of Gold + Silver/hr X hours saved) – (Cost of equipment)
REVENUE CALCULATIONS
Assumed Gold price 1149 $ / oz Au 36.94 $ / g Au
Assumed Silver price 15.54 $ / oz Ag 0.50 $ / g Ag
Potential gold revenue lost 3371.4 g/hr Au -$ 124,545
per hour potential
gold production lost
Potential silver revenue lost 19428.6 g/hr Ag -$ 9,707
per hour potential
silver production lost
Potential revenue loss -$ 134,252 per hour
REVENUE CALCULATIONS
Typical blowdown duration 24 hrs -$ 3,222,038 potential production loss
Blowdown with FlexStream 13 hrs -$ 1,745,271 potential production loss
Saving in blowdown time 11 hrs $ 1,476,767 early start up potential revenue

COSTS OF IMPLEMENTATION - COI Total
FlexStream RBV (complete)
8 ins ANS 600 + Actuator
$735,617 -$ 735,617
Piping modification 8 mtrs $ 450 / mtr -$ 3,600
C&I modifications & additions 0.5% -$ 3,678
Quench vessel modification $3,500 -$ 3,500
Quench vessel recertification (if req'd) $10,000 -$ 10,000
Labor 5 hrs @ $ 200 / hr -$ 1,000
Crain hire (if required) 2 hrs @ $ 700 / hr -$ 1,400
Total costs -$ 758,795
ROI
Saving $ 1,476,767 Per blowdown
Saving $ 7,383,835 Per 5 operations
COI $ 758,795
Total ROI $ 6,625,040 8.7 times
In addition to the hard numbers above other cost considerations that are not easily put into hard
numbers can also be counted :
The ROI is based up on a POX Gold & Silver production autoclave with a
pressure drop ratio for blowdown of between 0.02 to 1. In autoclaves
where the pressure drop ratio for autoclave blowdown is between 0.02
and 0.4 then ( e.g. a High Pressure Acid Leaching - HPAL Nickel
autoclave ) the FlexStream® trim can be designed with less pressure
letdown stages to achieve the same outcome.
Shift patterns can also be scheduled more efficiently to accommodate the
blowdown of the autoclave whether that is for a POX or HPAL installation.
Installation Update – April 2015
The client that installed the first FlexStream® RBV has since installed an additional 3 valves so each
of the autoclaves have this applied and an additional customer in South America has installed and is
currently commissioning a 3 in ANSI 600 FlexStream® RBV.

PROCESS BLOCK FLOW - Typical Pressure Oxidation
Line legend:
Gaseous
Solution
Slurry
Slurry Feed /
Concentrate Feed
Process
Water
Demin water
Steam
99.9%
Oxygen
Limestone
Milk of
Lime0.01%
Flocculant
Downstream
consumers
Tailings
Treatment
Process / Wash
Water
o/f
u/f
Scrub
Solution
POX Feed
Storage
POX
Autoclave(s)
Clave Vent
Scrubbing
Flash &
Splash
Vessels
Effluent
Thickening
BFS
Redissolution
Effluent
Treatment
bleed
To Downstream
Processing
Steam Generation
Filtered
Raw water
Demin Water
Plant
Cooling/Flush
Water
Oxygen Plant
Process
Water
o/f
Counter
Current
Decantation
Limestone
Neutrali-
sation
Limestone
POX Feed
Density
Adjustment
Downstream
processing (via
Reboiler)
Cogeneration
Area 4600
OR
Power supply
into grid
Milk of
Lime
u/f

PROCESS BLOCK FLOW - Typical Pressure Oxidation
Slurry Feed /
Concentrate Feed
Cooling/Flush
Water
Steam
99.9%
Oxygen
Atmosphere

Rapid Autoclave Blowdown 04.15doc

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