chemical engineering: Compressor

Showing posts with label Compressor. Show all posts

Sunday, August 8, 2010

Compression Prediction Gap & Highlights

Earlier post "Compression Prediction - Compressor Vendor, GPSA & HYSYS" has presented equation in determining Polytropic head, polytropic exponent, gas horse power and compressor discharge temperature.

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From data presented, two main findings are (i) GPSA method may be used but shall keep in mind GPSA potentially overpredicted discharge temperature. This potential results over conservative design and excessive cooling required. (ii) HYSYS prediction is using rigorous method which adjusting prediction rigorously and within a good range of prediction. This post will look at the influence of compressibility (z) on predicted temperature difference for several ordinary components.

Check Case Basis

Following tabulating the basis of compression calculation using GPSA and HYSYS.

1) Component used : Methane (C1), Ethane (C2), Propane (C3) and Iso-Butane (i-C4)

2) Suction temperature fixed at 45 degC for all calculation

3) Suction pressure range : 1, 3, 4.5, 5, 10, 20, 50 barg

4a) Discharge pressure for Methane (C1) case : 4, 8, 15, 30, 55 & 150 barg

4b) Discharge pressure for Ethane (C2) case : 4, 8, 15, 30, 55 & 150 barg

4c) Discharge pressure for Propane (C3) case : 4, 8, 15 & 30 barg

4d) Discharge pressure for Is-Butane (I-C4) case : 4, 8 & 15 barg
5) Polytorpic efficiency artificial set at 73% for all cases

Results
Calculation results shows that
i) GPSA predicted discharge temperature (Td) consistently higher than HYSYS rigorous prediction.
ii) As discharge pressure (Pd) increase, discharge compressibility (Zd) decrease consistently.
iii) As compressibility decrease (Zd) , the discharge temperature difference / gap (dT) increase significantly. See following chart.

iv) Considering discharge temperature difference / gap (dT) of 10 degC as limit, the compressibility is limited to 0.90 for discharge pressure 8 barg (and below).
v) Considering discharge temperature difference / gap (dT) of 10 degC as limit, the compressibility is limited to 0.96 for discharge pressure 15 barg (and below).
v) As discharge pressure increase above 15 barg, discharge temperature difference / gap (dT) will possibly higher than of 10 degC limit.

Highlights

i) Pressure lower than 15 barg, the GPSA and HYSYS prediction are considered acceptable.
ii) Once pressure higher than 15 barg, GPSA can severely overpredicts discharge temperature. Shall consider to use rigorous compression calculation (like HYSYS).

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Sunday, August 1, 2010

Compression Prediction - Compressor Vendor, GPSA & HYSYS

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Compressor is commonly used to compress gas and vapor to higher delivery pressure. Energy is supplied to the compressor to develop compression head. Part of the energy is lost when energy is transferred via shaft and part of energy lost due to compression activity. Energy lost via shaft will convert to vibration and noise. Energy lost due to compression activity (instead of carry out compression work) will turn to fluid internal energy of fluid. As fluid internal energy is increased, temperature of fluid will rise. How much energy is lost to compression activity ? How much internal energy is increased and how fluid temperature is increased ? All this relates to one well known parameter in compression field, Polytropic efficiency.

There are two paths compression is carried out :

1. isentropic reversible path - a process during which there is no heat added to or removed from the system
and the entropy remains constant, pv^k = constant
2. polytropic reversible path - a process in which changes in gas characteristics during compression are considered, pvⁿ = constant

One shall take note that most compressors operate along a polytropic path but approaches the isentropic. Most compressor will use polytropic efficiency to account for true behavior.

Compression following polytropic path,

Polytropic head

where
Z_avg = Average compressibility factor
T_s = Suction temperature (degK)
M = Molecular weight
n = polytropic exponent
P_d = Discharge pressure (bara)
P_s = Suction pressure (bara)

Polytropic exponent (n) can be calculated base on following equation

where
k = isentropic exponent
n_p = Polytropic efficiency

Gas Horse Power,

where
W = gas flowrate (kg/h)

Compressor discharge temperature

where
T_d = Discharge temperature (K)
T_s = Suction temperature (K)

Above equations were extracted from GPSA section 13.

Recent compression studies using several cases to find compressor gas horse power and discharge temperature with specific polytropic efficiency. The studies have used

GPSA method (as tabulated above)
HYSYS

to estimate compressor gas horse power and discharge temperature. Results from several international compressor suppliers.

Case	Items	Supplier	GPSA	HYSYS
1a	Discharge temperature(degC)	117.6	117.5	117.2
	Gas Horse Power (kW)	2696.0	2678.4	2690.1

1b	Discharge temperature(degC)	121.2	121.0	120.6
	Gas Horse Power (kW)	2877.0	2858.4	2871.4

2a	Discharge temperature(degC)	117.2	117.5	116.5
	Gas Horse Power (kW)	10828.0	10740	10651.4

2b	Discharge temperature(degC)	88.0	88.0	87.4
	Gas Horse Power (kW)	4628.0	4575.1	4532.6

3a	Discharge temperature(degC)	124.1	140.9	124.7
	Gas Horse Power (kW)	8966.0	9147.8	9039.0

3b	Discharge temperature(degC)	85.0	117.9	86.1
	Gas Horse Power (kW)	3682.0	3798.0	3736.6

4a	Discharge temperature(degC)	122.5	139.5	125.8
	Gas Horse Power (kW)	9090.4	9210.7	9162.0

4b	Discharge temperature(degC)	86.3	120.7	87.1
	Gas Horse Power (kW)	3829.6	3926.7	3859.7

5a	Discharge temperature(degC)	123.2	142.3	125.6
	Gas Horse Power (kW)	9104.0	9262.2	9149.5

5b	Discharge temperature(degC)	95.7	121.0	87.2
	Gas Horse Power (kW)	4293.0	3937.8	3844.0

Several observations :
i) HYSYS consistently predict discharge temperature similar to compressor supplier results.
ii) GPSA overpredict discharge temperature for several cases.
iii) HYSYS & GPSA predict gas horse power proximity to compressor supplier results with HYSYS in better prediction.

Above results give us some indication that
i) GPSA method may be used but shall keep in mind GPSA potentially overpredicted discharge temperature. This potential results over conservative design and excessive cooling required.
ii) HYSYS prediction is using rigorous method which adjusting prediction rigorously and within a good range of prediction.

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Sunday, November 30, 2008

Improved Manual Method for Settle Out Condition Estimation

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In earlier discussion "Simple Manual Method for Settle Out Condition Estimation", a manual method has been can be considered without using any Process Simulator. This method basically utilising universal gas law (PV=znRT) with the following basis and assumption :

Vapor only system
No condensation during the process
Compressibility factor assumed same for condition before and after settle-out and assumed (z=1)
Limited fluid with molecular weight is similar range. Higher the different, higher the deviation

In this method (T x n method), the settle out temperature is calculated using this equation :

T_s = Sum (n₁ x T₁ + n₂ x T₂ + n₃ x T₃ +...) / n_s

This method (T x n method) has a particular known issue where it does not considered the impact of Molecular Weight (MW). In the event molecular weight of each sections are different, the calculated settle temperature will not change with molecular weight. In this post, an improved method is introduced to use same derivation per T x n method, however it includes MW as part the calculation. It basically use mass (m) to replace mole (n) which simply name as T x m method.

Derivation
A system consists of n-section with pressure (P_i, kPag), temperature (Ti, K), Molecular weight (MW_i), physical volume (V_i, m3) for i-section.

Number of mass in each i-section (before settle-out),

m_i = (P_i x V_i x MW_i) / (z_i x R x T_i).....[1]

Total mass (after settle-out),

m_s = Sum (m₁ + m₂ + m₃...).....[2]

Total volume (after settle-out),

V_s = Sum (V₁ + V₂ + V₃...).....[3]

Volume at normal condition (P_i,n = 1.01325 bar & Ti,_n = 273.15 K) for each i-section (before settle-out)

V_i,n = (P_i x V_i / T_i) /(P_i,n / T_i,n)......[4]

Total volume at normal condition (after settle-out)

V_s,n = Sum (V_1,n + V_2,n + V_3,n...).....[5]

m_i x T_i for each i-section (before settle-out),

m_i x T_i = (P_i x V_i x MW_i) / (z_i x R).....[6]

Total m_s x T_s (after settle-out),

m_s x T_s = Sum (m₁ x T₁ + m₂ x T₂ + m₃ x T₃ +...).....[7]

Thus, From [7] and [2],
Settle-out temperature (T_s),

T_s = Sum (m₁ x T₁ + m₂ x T₂ + m₃ x T₃ +...) / m_s .....[8]

Settle-out pressure (P_s),

P_s = (1.01325) x (V_s,n/ V_s) x (T_s / 273.15)......[9]

Case Study
There are five sets of system with each system has 3 section will be settled-out. The five set of fluid will have same pressure and temperature prior to settle out, however the composition of each section will be difference as follow :

Composition Set 1 :
Section 1 : Methane : 100%
Section 2 : Methane : 100%
Section 3 : Methane : 100%

Composition Set 2 :
Section 1 : Ethane : 100%
Section 2 : Methane : 100%
Section 3 : Methane : 100%

Composition Set 3 :
Section 1 : Ethane : 100%
Section 2 : Methane : 100%
Section 3 : Methane : 50%, Ethane : 20%, Propane : 30%

Composition Set 4 :
Section 1 : Propane : 100%
Section 2 : Propane : 100%
Section 3 : Methane : 50%, Ethane : 20%, Propane : 30%

Composition Set 5 :
Section 1 : Propane : 100%
Section 2 : Propane : 100%
Section 3 : Ethane : 40%, Propane : 60%

Image below display the results of HYSYS Settle-out using method in "Simple Method For Compressor Settle Out (Vapor Only) Using HYSYS", T x n and T x m methods.

Above results show
i) Settle-out pressure and temperature will not change with composition.
ii) if the composition are close between each sections, both T x n and T x m may be used.
iii) Accuracy for T x m are typically higher than T x n methods.

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Saturday, November 22, 2008

Simple Manual Method for Settle Out Condition Estimation

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Compression system shutdown, block-in, settle out follow by system blowdown is common in any oil and gas, refinery and LNG production facilities. A methodology using HYSYS simulation software has been presented in "Simple Method For Compressor Settle Out (Vapor Only) Using HYSYS" and this method is particularly applicable to vapor only system. In order to handle system contains vapor and liquid, the method has been extended "Adjusted Method For Compressor Settle Out (with Vapor & Liquid) Using HYSYS". Some engineers asked a question, this method is applicable during detailed design. Nevertheless, is there any simpler and manual method that may be used for quick estimation during proposal and/or conceptual stage ? Yes... Following is a simple manual method may be considered.

This method is utilising universal gas law (PV=znRT) with the following basis and assumption :

Vapor only system
No condensation during the process
Compressibility factor assumed same for condition before and after settle-out and assumed (z=1)
Limited fluid with molecular weight is similar range. Higher the different, higher the deviation

A system consists of n-section with pressure (P_i, kPag), temperature (Ti, K), Molecular weight (MW_i), physical volume (V_i, m3) for i-section.

Number of mole in each i-section (before settle-out),

n_i = (P_i x V_i) / (z_i x R x T_i).....[1]

Total mole (after settle-out),

n_s = Sum (n₁ + n₂ + n₃...).....[2]

Total volume (after settle-out),

V_s = Sum (V₁ + V₂ + V₃...).....[3]

Volume at normal condition (P_i,n = 1.01325 bar & Ti,_n = 273.15 K) for each i-section (before settle-out)

V_i,n = (P_i x V_i / T_i) /(P_i,n / T_i,n)......[4]

Total volume at normal condition (after settle-out)

V_s,n = Sum (V_1,n + V_2,n + V_3,n...).....[5]

n_i x T_i for each i-section (before settle-out),

n_i x T_i = (P_i x V_i) / (z_i x R).....[6]

Total n_s x T_s (after settle-out),

n_s x T_s = Sum (n₁ x T₁ + n₂ x T₂ + n₃ x T₃ +...).....[7]

Thus, From [7] and [2],
Settle-out temperature (T_s),

T_s = Sum (n₁ x T₁ + n₂ x T₂ + n₃ x T₃ +...) / n_s .....[8]

Settle-out pressure (P_s),

P_s = (1.01325) x (V_s,n/ V_s) x (T_s / 273.15)......[9]

Case Study
A methane compression system with the following conditions.

Suction :
Pressure, P₁= 5 barg
Temperature, T₁= 50 degC
Molecular weight, MW = 16.0429
Physical Volume, V₁= 1 m³

Compressor discharge (Hot) :
Pressure, P₂= 15 barg
Temperature, T₂= 150 degC
Molecular weight, MW = 16.0429
Physical Volume, V₂= 1 m³

Cooler discharge (Cool) :
Pressure, P₃= 15 barg
Temperature, T₃= 50 degC
Molecular weight, MW = 16.0429
Physical Volume, V₃= 1 m³

Using method as proposed in "Simple Method For Compressor Settle Out (Vapor Only) Using HYSYS", the settle out pressure and temperature are 11.73 barg and 86.3 degC. (see below image).

Using manual method as proposed above (program in Excel), the settle out pressure and temperature are 11.67 barg and 85.7 degC. (see below image).

The percentage error are 0.5% and 0.7% for pressure and temperature respectively.
This shown the method is reasonable.

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Tuesday, November 11, 2008

Adjusted Method For Compressor Settle Out (with Vapor & Liquid) Using HYSYS

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Since the release of "Simple Method For Compressor Settle Out Using HYSYS", some readers of Chemical & Process Technology raised a question. The proposed method has considered all in VAPOR, how shall this method apply in case of present of VAPOR and LIQUID in Compressor Suction drum and Air Cooler Downstream ?

It is correct that the simple method is simplified version for VAPOR only Settle out condition. Nevertheless, minimum adjustment to the method enable the method to be used for condition with VAPOR and LIQUID. First refer to following image.

Step 1 : Compressor Suction. Estimate physical volume of vapor (Vv1) and liquid (Vl1).

Step 2a : Compressor Suction. Separate Compressor Suction Inlet (stream 1) with 2-phases (vapor & Liquid) with Separator unit operation.

Step 2b : Copy Compressor Suction Inlet Vapor (stream 2) condition and composition to a new stream Actual Compressor Suction Inlet Vapor ((stream 4). Adjust Mass flow of this stream until the Actual Volumetric Flow equal to Vv1.

Step 2c : Copy Compressor Suction Inlet Liquid (stream 3) condition and composition to a new stream Actual Compressor Suction Inlet Liquid (stream 5). Adjust Mass flow of this stream until the Actual Volumetric Flow equal to Vl1.

Step 2d : Mix Actual Compressor Suction Inlet Vapor (stream 4) and Actual Compressor Suction Inlet Liquid (stream 5) to form Compressor Suction (stream 6).

Normally the Compressor Discharge stream is superheated and no liquid is expected.

Step 3 : Air Cooler Downstream. Estimate physical volume of vapor (Vv3) and liquid (Vl3).

Step 4a : Air Cooler Downstream. Separate Air Cooler Downstream Outlet (stream 7) with 2-phases (vapor & Liquid) with Separator unit operation.

Step 4b : Copy Air Cooler Downstream Outlet Vapor (stream 8) condition and composition to a new stream Actual Air Cooler Downstream Outlet Vapor (stream 9) . Adjust Mass flow of this stream until the Actual Volumetric Flow equal to Vv3.

Step 4c : Copy Air Cooler Downstream Outlet Liquid (stream 10) condition and composition to a new stream Actual Air Cooler Downstream Outlet Liquid (stream 11). Adjust Mass flow of this stream until the Actual Volumetric Flow equal to Vl3.

Step 4d : Mix Actual Air Cooler Downstream Outlet Vapor (stream 9) and Actual Air Cooler Downstream Outlet Liquid (stream 11) to form Air Cooler Downstream (stream 12).

The remaining steps are same as "Simple Method For Compressor Settle Out Using HYSYS" by adjusting Settle Out Cond Actual Volumetric Flow same as Vv1+Vl1+V2+Vv3+Vl3.

Thursday, November 6, 2008

Simple Method For Compressor Settle Out Using HYSYS

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Compressor emergency blowdown is commonly initiated after compressor system shutdown. The compressor shutdown will also lead to Compressor System Suction shutdown valve (SDV) and Discharge SDV to shut close and the anti-surge and/or capacity control valve open. Compressor discharge with high pressure and hot, air cooler and downstream high pressure and cold and suction vessel low pressure and cold will be settled-out prior to blowdown.

How to estimate the settle out condition ?

Settle-out of compressor system can be conducted using HYSYS. The main concept is constant mass and volume before and after settle-out. It is assumed the settle out is carried out adiabatically.

Following are some steps can be used to obtain the settle-out condition. Let start with following image.

Step 1 : Calculate physical volume of Compressor Suction (V1), Compressor Discharge (V2) and Air Cooler Downstream (V3).

Step 2 : Setup the system as per above image by correct Compressor Suction, Compressor Discharge and Air Cooler Downstream operating condition and composition.

Step 3 : Adjust Compressor Suction, Compressor Discharge and Air Cooler Downstream Mass flow until the respective Actual Volumetric Flow same as V1, V2 and V3.

Step 4 : Set Balance (BAL-1) for Mole & Heat

Step 5 : Adjust Settle-out Cond stream pressure until the Actual Volumetric Flow same as V1+V2+V3.

Thus, the settle out condition can be extracted from the Settle-out Cond stream.

Do you aware of other method ? Why not share with us ?

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Tuesday, October 21, 2008

Anti-surge Control (ASC) or Capacity Control (CC) Valve in Vertical Upward Run ?

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Centrifugal compressor is widely used to increase process fluid pressure head to meet process requirement. All centrifugal compressors are equipped with Anti-surge control valve (ASCV) for equipment protective purpose. In many events, This valve may also serve as capacity control valve in order to maintain a specific process parameter.

One of the question being discussed :
Should we locate compressor Anti-surge control (ASC) or Capacity Control (CC) Valve in vertical upward run ?

Good engineering practice is to install anti-surge control valve in horizontal run and no low pocket along the inlet and outlet of the control valve to avoid any possible liquid (or solid) accumulation which possibly lead to issues like corrosion, liquid slug, liquid freezing, solid plugging, etc.

Liquid Condensation & Accumulation Causing Corrosion

When ASCV / CCV in close position, vapor with mist liquid may diffuse along the inlet and outlet of ASCV/CCV. Due to heat loss to ambient and/or mist coalescence, mist is possible condensed and accumulated in low pocket and/or downstream of ASCV/CCV downstream on vertical upward run. If the fluid is wet and contains corrosive compounds i.e. Hydrogen Sulfide (H2S), Carbon Dioxide (CO2), low pocket and/or upward run where liquid is accumulated will experience acid corrosion. Pitting and crevice corrosion may be experienced.

Hydrate formation and/or Water Freezing

A fluid with hydrate former and wet, when there is liquid accumulated due to external ambient cooling, there is potential risk of hydrate formation and water freezing. This potential partially or totally clog the recycle line.

Fluid Possibly Crystallization and Solidify
A fluid contain compound possible crystallize and solidify when it is cooled by ambient, when the fluid is cooled by ambient in the stagnant section in recycle line, the fluid is possibly accumulated at low pocket and/or vertical upward run, downstream of ASCV/CCV and crystallized or solidified and plugged the recycle line

Liquid Slugging flow & Induced Vibration

ASCV/CCV is normally closed and liquid is accumulated along the recycle line due to external cooling. In the event of ASCV / CCV open to recycle vapor at compressor discharge back to suction, vapor will push the liquid column flow along the recycle line. This liquid column will be knocking at the ASCV/CCV, bend and tee. Severe slugging and piping vibration may occur and potential damage ASCV/CCV and line.

Thus, it is always recommended to install ASCV / CCV in horizontal run and no low pocket along the inlet and outlet of the control valve that potentially promote corrosion, slugging flow, liquid freezing and solid plugging.

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Saturday, October 18, 2008

Combine Anti-surge control (ASC) & Capacity Control (CC) Functions ?

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Control valve is widely used in Oil & gas, refinery, Petrochemical and chemical plant for control purposes of operating parameters i.e temperature, pressure, level, etc. Several articles as compiled in "Useful Documents Related to Control Valve" are pretty useful to many process and chemical engineers.

Centrifugal compressor is widely used to increase process fluid pressure head to meet process requirement. All centrifugal compressors are equipped with Anti-surge control valve (ASCV) for equipment protective purpose. In many events, This valve may also serve as capacity control valve in order to maintain a specific process parameter.

Anti-surge control valve (ASCV) is one of required equipment protective function for centrifugal compressor. It is used to protect centrifugal compressor from running in surge region, a phenomenon where centrifugal compressor discharge pressure high enough to results reverse flow follow by severe vibration in the compressor chamber. The Anti-surge control valve is recycling discharge gas back to suction to minimize differential pressure across the centrifugal compressor.

Capacity Control valve (CCV) is normally used to maintain a process parameter of a system by recycling excess gas back to compressor suction. Typical process parameters of a system are fix suction pressure, fix discharge pressure and fix differential pressure.

One of the common question raised is :
Should we provide dedicated control valve for anti-surge and capacity control purpose or combine function control valve ?"

There are several factors lead to separate control valve for dedicate function.

Control Valve Characteristic

This is one of the most important factor to determine if separate control valve is required. For an anti-surge control purpose, the best control valve characteristic is quick opening. However, for capacity control purpose, an equal percentage is mostly used. Thus, this could easy lead to separate control valve with different characteristics used for dedicated function. However, in many occasion, the valve with equal percentage may also serve the purpose of anti-surge control. This is very much subject to compressor characteristic, system operating pressure, system volume, etc. Only way to prove if a single control serve two purpose is Dynamic simulation.

Safety
In many event, the Anti-surge control system together with anti-surge control valve are within a commercial package of compressor. The purpose is to have single point coordination and responsibility and to avoid unnecessary interference. However, using control valve dedicated to Anti-surge control purpose for capacity control purpose would lead to interference and additional signal managing the control valve. Some company safety principle do not allow an protection function used as control function as well. Thus, a separate / dedicate control valve is used. In many recent serious improvement in technology and working method, one control valve serving dual purposes are widely implemented recently.

Wear & Tear
For a control valve serving dual purposes, the control valve may continues in service and this promote wear and tear of the control valve. This would increase the downtime and availability of this control valve. Doubling the control valves will recover the availability again.

Cost
Dedicated control valve for ASCV and CCV is obviously require higher capital investment.

Used of dedicated control valve for anti-surge and capacity control purpose or combine function control valve is subject to case-by-case basis. No one key fit all locks...

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