HP per MMSCFD Estimation Fundamentals

1. Overview

HP per MMSCFD is the most widely used quick-estimation metric in the midstream gas industry. It allows engineers, operations personnel, and commercial teams to rapidly size compressor packages without detailed thermodynamic calculations. While not a substitute for rigorous engineering, these estimates are invaluable for feasibility studies, budgeting, and field decisions.

Feasibility Studies

Budget Estimates

Capital cost = f(installed HP); quick screening

Field Operations

Quick Sizing

Can existing unit handle new well volumes?

Rental Units

Fleet Selection

Match available HP to field requirements

Accuracy

+/- 10-20%

Suitable for planning; confirm with calcs

When to use detailed calcs: HP/MMSCFD estimates are for preliminary screening only. Final compressor selection requires rigorous thermodynamic calculations with actual gas analysis, site conditions, and equipment performance curves.

2. Rules of Thumb

The following HP/MMSCFD factors are based on natural gas with specific gravity 0.60-0.70 (MW = 17-20), suction temperature of 80-100 F, and typical reciprocating compressor efficiency of 82-85%.

GPSA-Based HP/MMSCFD Factors

Compression Ratio	HP/MMSCFD	Application	Typical Service
r = 1.2	8-12	Booster	Pipeline recompression
r = 1.5	18-25	Low-ratio boost	Station-to-station
r = 2.0	30-40	Moderate	Gathering, gas lift
r = 2.5	45-55	Standard	Central facility
r = 3.0	55-70	Standard	Field compression
r = 3.5	70-85	Moderate-high	Wellhead compression
r = 4.0	80-100	High ratio	Gas plant inlet
r = 5.0	95-120	Multi-stage	High-pressure injection
r = 6.0	110-140	Multi-stage	Gas storage cycling
r = 8.0	130-170	Multi-stage	Vapor recovery to pipeline
r = 10.0	150-200	Multi-stage	Low-pressure gathering

Quick Estimation Formula: BHP = (Flow in MMSCFD) x (HP/MMSCFD factor) Example: 10 MMSCFD at r = 3.0: BHP = 10 x 65 = 650 HP (use 800 HP driver) Driver Sizing Rule: Select driver HP = BHP x 1.10 to 1.15 (10-15% margin for fouling, altitude, ambient temp)

Common Field Shorthand

The "22-per-ratio" Rule: For natural gas (SG ~ 0.65), a quick approximation: HP/MMSCFD ~ 22 x ln(r) x (1 + 0.1 x ln(r)) This gives: r = 2.0: 22 x 0.693 x 1.069 = 16.3 (~35 with losses) r = 3.0: 22 x 1.099 x 1.110 = 26.8 (~60 with losses) r = 4.0: 22 x 1.386 x 1.139 = 34.7 (~80 with losses) Note: Multiply by 2.0-2.3 to account for mechanical losses, altitude, and safety margin for practical use.

3. Compression Ratio Effect

The compression ratio is the dominant factor in HP/MMSCFD. Power increases non-linearly with ratio, following the isentropic work equation. Understanding this relationship helps engineers quickly assess whether a proposed compression scheme is feasible.

Theoretical Basis (from GPSA): HP/MMSCFD = C x [k/(k-1)] x T1 x Z x [r^((k-1)/k) - 1] / eta Where C = unit conversion constant The key term is: r^((k-1)/k) - 1 This grows sub-linearly with r: r = 2.0: 2.0^0.2126 - 1 = 0.160 r = 3.0: 3.0^0.2126 - 1 = 0.265 r = 4.0: 4.0^0.2126 - 1 = 0.345 r = 6.0: 6.0^0.2126 - 1 = 0.470 r = 10: 10^0.2126 - 1 = 0.636 Power per unit flow roughly doubles when ratio triples.

Ratio Limits and Staging Decisions

Overall Ratio	Stages	HP/MMSCFD (total)	vs. Single-Stage
r = 4.0	1	85-100	Baseline
r = 4.0	2	75-90	10-15% savings
r = 9.0	2	120-145	Baseline (2-stage)
r = 9.0	3	105-130	10-15% savings
r = 16.0	2	175-210	Baseline (2-stage)
r = 16.0	3	150-180	12-18% savings
r = 16.0	4	140-170	15-20% savings

Suction Pressure Effects

At the same compression ratio, lower suction pressures require more HP/MMSCFD because the gas is less dense, requiring larger swept volume and more work per unit of standard volume.

P1 (psia)	P2 (psia)	Ratio	HP/MMSCFD	Notes
25	75	3.0	70-85	Low-pressure gathering
100	300	3.0	60-70	Standard gathering
300	900	3.0	55-65	Pipeline compression
500	1500	3.0	55-65	High-pressure service

Low-pressure penalty: Compressors operating below 50 psia suction require 10-20% more HP/MMSCFD than the standard tables suggest. This is due to higher Z-factor deviation, lower volumetric efficiency, and larger required cylinder bore.

4. Gas Properties Impact

Standard HP/MMSCFD factors assume natural gas with SG = 0.60-0.70 (MW = 17-20). Different gas compositions significantly affect power requirements through changes in specific heat ratio (k), compressibility (Z), and molecular weight (MW).

Specific Gravity Correction

SG Correction Factor: HP_actual = HP_table x F_SG Where F_SG varies with gas composition: SG = 0.55 (lean gas): F_SG = 0.95-1.00 SG = 0.65 (standard): F_SG = 1.00 (reference) SG = 0.75 (rich gas): F_SG = 0.95-1.00 SG = 0.85 (very rich): F_SG = 0.90-0.95 SG = 1.00 (heavy gas): F_SG = 0.85-0.90 Why heavier gas needs less HP: Heavier gases have lower k values, which reduces the isentropic work term [r^((k-1)/k) - 1]. The effect of lower k outweighs the mass flow increase from higher MW.

Gas Composition Effects

Gas Component	Effect on k	Effect on Z	HP/MMSCFD Impact
Methane (C1)	k = 1.31	Z ~ 0.99	Reference; highest HP for hydrocarbons
Ethane (C2)	k = 1.19	Z ~ 0.95	10-15% less HP than pure C1
Propane (C3)	k = 1.13	Z ~ 0.90	15-25% less HP; watch for liquids
CO2	k = 1.29	Z ~ 0.85	Lower Z partially offsets lower k
H2S	k = 1.33	Z ~ 0.90	Similar to C1; corrosion limits apply
N2	k = 1.40	Z ~ 1.00	10-15% more HP than natural gas
H2	k = 1.41	Z ~ 1.00	Highest HP/MMSCFD of any gas

Temperature Correction

Suction Temp (F)	Correction Factor	Notes
60	0.96	Cool climate / after cooling
80	0.98	Standard reference (GPSA)
100	1.00	Typical field conditions
120	1.02	Hot climate / inadequate cooling
140	1.05	After 1st stage without intercooler

Combined corrections: For non-standard conditions, multiply: HP = HP_table x F_SG x F_temp x F_altitude. Each correction is typically small (2-10%), but combined they can shift the estimate by 15-25%.

5. Multi-Stage Power Estimation

For multi-stage compression, calculate HP/MMSCFD for each stage individually using the per-stage ratio, then sum them. This accounts for intercooling benefits and the different conditions at each stage suction.

Multi-Stage HP Estimation: Total HP = Sum of (HP/MMSCFD)_i x Flow_i for each stage i For equal ratio staging with perfect intercooling: r_stage = R_overall^(1/N) HP_total ~ N x HP_per_stage(r_stage) Example: 3-Stage, R = 27 r_stage = 27^(1/3) = 3.0 HP/MMSCFD per stage ~ 65 Total HP/MMSCFD ~ 3 x 65 = 195 vs. single stage (theoretical): HP/MMSCFD ~ 280-320 Savings: 35-40% from intercooling

Unequal Ratio Distribution

In practice, equal ratios per stage are not always optimal. Considerations include available cylinder sizes, interstage condensation, and driver loading. However, deviating more than 15% from equal ratios per stage increases total power and discharge temperatures.

Configuration	Stage 1 Ratio	Stage 2 Ratio	Total HP/MMSCFD	Penalty
Equal (optimal)	3.0	3.0	130	0% (baseline)
Slightly unequal	3.5	2.57	133	+2%
Moderately unequal	4.0	2.25	138	+6%
Significantly unequal	5.0	1.80	145	+12%

Overall ratio R = 9.0. Natural gas, k = 1.27, T1 = 100 F, IC to 120 F.

Altitude and Ambient Corrections

Altitude Derating: Engine-driven compressors lose power at altitude: Sea level: 100% rated HP 2,000 ft: 96% rated HP 4,000 ft: 93% rated HP 5,000 ft: 91% rated HP 6,000 ft: 89% rated HP 8,000 ft: 84% rated HP Required HP at altitude: HP_required = HP_sea_level / (derating factor) Example: 800 HP needed at sea level, site at 5,000 ft HP_driver = 800 / 0.91 = 879 HP --> select 900 HP unit Electric motor drives: No altitude derating for power (but check motor cooling above 3,300 ft per NEMA MG1).

Capital cost rule of thumb: Installed cost for reciprocating compressor packages ranges from $800-$1,500 per HP for standard natural gas service. A 1,000 HP package typically costs $1.0-$1.5 million installed, including engine/motor, coolers, scrubbers, and controls.

6. Worked Examples

Example 1: Field Gathering Compression

Given: Flow: 15 MMSCFD natural gas (SG = 0.65) P1 = 150 psia, P2 = 500 psia T1 = 90 F, elevation = 3,000 ft Step 1: Compression ratio r = 500/150 = 3.33 Step 2: HP/MMSCFD from table At r = 3.33: interpolate between r=3.0 (65) and r=3.5 (78) HP/MMSCFD ~ 65 + (78-65) x (0.33/0.50) = 73.6 ~ 74 Step 3: Base HP BHP = 15 x 74 = 1,110 HP Step 4: Altitude correction At 3,000 ft: derating = 0.945 Required driver HP = 1,110 / 0.945 = 1,175 HP Step 5: Driver selection Select 1,340 HP engine (next standard size with 15% margin) or 1,250 HP electric motor (no altitude derating) Result: 1,110 BHP / 1,340 HP engine package

Example 2: Vapor Recovery Unit

Given: Flow: 2 MMSCFD tank vapor (SG = 1.10, rich gas) P1 = 2 psig = 16.7 psia, P2 = 75 psia T1 = 100 F Step 1: Compression ratio r = 75/16.7 = 4.49 Step 2: Staging decision r = 4.49 > 4.0; consider two stages r_stage = 4.49^(1/2) = 2.12 P_inter = sqrt(16.7 x 75) = 35.4 psia Step 3: HP/MMSCFD per stage At r = 2.12: HP/MMSCFD ~ 38 (from table) But heavy gas correction: F_SG = 0.88 Adjusted: 38 x 0.88 = 33.4 per stage Step 4: Total HP BHP = 2 x (33.4 + 33.4) = 134 HP Add 10% for low-pressure penalty: 134 x 1.10 = 147 HP Step 5: Select unit 200 HP package (standard rental size) Result: ~150 BHP / 200 HP package

Quick Reference: Common Applications

Application	Typical Ratio	HP/MMSCFD	Typical Unit Size
Pipeline booster	1.3-1.8	15-30	500-3,000 HP
Gas gathering	2.5-4.0	50-90	200-1,500 HP
Gas lift	3.0-6.0	60-130	300-2,000 HP
Gas plant inlet	2.0-4.0	40-90	500-5,000 HP
Vapor recovery	4.0-15.0	90-200	100-500 HP
Gas storage injection	3.0-10.0	60-180	2,000-10,000 HP
CNG fueling	10-30	200-350	50-500 HP

HP per MMSCFD Estimation

20-25 HP/MMSCFD

55-70 HP/MMSCFD

90-120 HP/MMSCFD

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