Maxx Engineering & Maintenance Co., Ltd.

01/09/2026

AWS D1.6 vs AWS D1.1 – Welding Code Comparison

AWS D1.1 and D1.6 are structural welding codes developed by the American Welding Society (AWS). While AWS D1.1 governs the welding of carbon and low-alloy steels, AWS D1.6 focuses on stainless steel structures where corrosion resistance is crucial. Both codes ensure weld quality, safety, and compliance in different industrial applications.

Key Takeaway:

A. AWS D1.6 ensures corrosion-resistant weld integrity in stainless structures.

B. AWS D1.1 ensures strength and toughness in carbon steel structures.

01/09/2026

28/08/2026

One 90° elbow or two 45° elbows — which would you choose?

A small change in duct or piping geometry can have a measurable impact on system performance.

Using two 45° elbows instead of one 90° elbow can help reduce flow disturbance and pressure losses, potentially improving pump/fan efficiency.

But engineering is always a balance.

You may gain in:
→ Flow performance
→ Lower pressure losses
→ Better system efficiency

While potentially increasing:
→ Material cost
→ Installation cost
→ Required space

The real question isn't simply “Which option is better?”

It's:

“Is the additional cost and space justified by the performance improvement?”

That's where engineering judgment comes in.

💬 What would you choose for your project: 1 × 90° elbow or 2 × 45° elbows — and why?

Note: The CFD animation featured in this post was shared from another creator. I'm using it because it provides an excellent visual demonstration of how turning vanes influence airflow.

27/08/2026

Shell side pressure drop with baffles.!

In a shell-and-tube heat exchanger, baffles are installed on the shell side to support the tubes and force the shell-side fluid to follow a controlled cross-flow path. This improves heat transfer, but it also increases pressure drop due to changes in flow direction, flow through baffle windows, and friction.

🔹 how baffles affect shell-side flow

As the shell-side fluid enters the exchanger, the baffles repeatedly redirect the flow across the tube bundle.

The flow pattern generally involves:

shell-side inlet → baffle window → cross-flow across tubes → next baffle window → repeated cross-flow → shell-side outlet

This repeated change in flow direction increases turbulence and improves heat transfer, but it also creates additional pressure losses.

🔹 major components of shell-side pressure drop

The total shell-side pressure drop can be considered from several contributions:

1️⃣ pressure drop across the baffle windows

Fluid accelerates as it passes through the available baffle-window area. This produces a pressure loss associated with the window velocity.

The infographic represents this contribution as:

ΔPᵥ = Gₛ² / (2ρₛ) × (1/Cᵥ² − 1)

where:

• Gₛ = shell-side mass velocity
• ρₛ = shell-side fluid density
• Cᵥ = window velocity coefficient

2️⃣ pressure drop due to cross-flow

After passing through a baffle window, the fluid flows across the tube bundle. Tube resistance and repeated flow redirection contribute significantly to pressure drop.

The cross-flow contribution can be represented by:

ΔPcf = 4 fᶜ Gₛ² Nᵦ / (ρₛ Dₑ)

where:

• fᶜ = cross-flow friction factor
• Nᵦ = number of baffle spaces
• Dₑ = equivalent diameter of the baffle-window flow passage

The equivalent diameter depends on the geometry of the shell and baffle window.

3️⃣ pressure drop due to shell friction

Additional pressure loss occurs because of friction along the shell-side flow path, including entrance and exit effects.

The infographic uses:

ΔPf = 2 fₛ Gₛ² (Lₛ + Lₑ) / (ρₛ Dₒ)

where:

• fₛ = shell friction factor
• Lₛ = shell length between the first and last baffle
• Lₑ = equivalent entrance and exit length
• Dₒ = shell outside diameter

🔹 correction factors

Real heat exchangers can have leakage paths and bypass streams that influence the actual pressure drop.

Therefore, correction factors may be applied:

ΔPₛ = (ΔPᵥ + ΔPcf + ΔPf) × Cᴸ × Cᴮ

where:

• Cᴸ = leakage correction factor
• Cᴮ = bypass correction factor

These factors help account for flow that does not follow the idealized cross-flow path.

🔸 Heat-transfer coefficient
🔸 Shell-side velocity
🔸 Baffle spacing
🔸 Baffle-window area
🔸 Tube-bundle geometry
🔸 Fluid properties
🔸 Leakage and bypass streams
🔸 Allowable pressure drop
🔸 Pumping/compressor power.

27/08/2026

𝗣𝗩 𝗘𝗹𝗶𝘁𝗲 𝗦𝗮𝗱𝗱𝗹𝗲 𝗟𝗶𝗺𝗶𝘁𝗮𝘁𝗶𝗼𝗻𝘀
• 𝗭𝗶𝗰𝗸 𝗙𝗼𝗿𝗺𝘂𝗹𝗮 𝗔𝘀𝘀𝘂𝗺𝗽𝘁𝗶𝗼𝗻𝘀
1. Cylindrical shell with hemispherical/torispherical heads.
2. Two saddles placed equidistant from vessel ends.
3. Saddle spacing ≤ 0.25L (where L = cylindrical shell length).
4. Uniform load distribution assumed.
• 𝗚𝗲𝗼𝗺𝗲𝘁𝗿𝘆 𝗥𝗲𝘀𝘁𝗿𝗶𝗰𝘁𝗶𝗼𝗻𝘀
1. Non‑standard head types, conical transitions, or irregular cutouts not modeled.
2. Saddles placed out of sequence (not left‑to‑right) trigger error checks.
• 𝗦𝘁𝗿𝗲𝘀𝘀 𝗔𝗻𝗮𝗹𝘆𝘀𝗶𝘀 𝗟𝗶𝗺𝗶𝘁𝘀
1. Relies on empirical methods (WRC 107/297/537).
2. Cannot capture localized stresses, plastic deformation, or complex load paths.
3. Limited treatment of thermal gradients and creep/fatigue.

✅ 𝗪𝗵𝗲𝗻 𝗦𝗽𝗲𝗰𝗶𝗮𝗹 𝗙𝗘𝗔 𝗜𝘀 𝗥𝗲𝗾𝘂𝗶𝗿𝗲𝗱
• Saddles 𝗻𝗼𝘁 𝗲𝗾𝘂𝗶𝗱𝗶𝘀𝘁𝗮𝗻𝘁 or spacing > 0.25L.
• 𝗟𝗮𝗿𝗴𝗲 𝗼𝗿 𝗵𝗲𝗮𝘃𝘆 𝘃𝗲𝘀𝘀𝗲𝗹𝘀 where bending moments exceed Zick’s scope.
• 𝗡𝗼𝗻‐𝘀𝘁𝗮𝗻𝗱𝗮𝗿𝗱 𝗴𝗲𝗼𝗺𝗲𝘁𝗿𝗶𝗲𝘀 (conical shells, offset saddles, irregular reinforcements).
• 𝗖𝗼𝗺𝗽𝗹𝗲𝘅 𝗹𝗼𝗮𝗱 𝗰𝗮𝘀𝗲𝘀 (thermal gradients, seismic/wind loads, nozzle interactions).
• 𝗖𝗿𝗶𝘁𝗶𝗰𝗮𝗹 𝘃𝗲𝘀𝘀𝗲𝗹𝘀 requiring ASME VIII‑2 Design‑by‑Analysis compliance.

27/08/2026

Thermal stresses calculation.!

Thermal stress is an important consideration in pipes, pressure vessels, heat exchangers, boilers, structural members, and other restrained equipment exposed to temperature changes.

When a metal member is heated, it naturally wants to expand. If its expansion is completely or partially restricted by supports, anchors, connected piping, or surrounding equipment, thermal stress develops.

🔹 Free thermal expansion

If the member is free to expand:

δ = α L ΔT

where:
• δ = thermal expansion
• α = coefficient of thermal expansion
• L = original length
• ΔT = temperature change

🔹 Thermal stress in a fully restrained member

For simple axial restraint:

σₜₕ = E α ΔT

where:
• E = Young's modulus
• α = coefficient of thermal expansion
• ΔT = temperature difference

🔹 Thermal force

The resulting axial force can be estimated as:

Pₜₕ = σₜₕ A = E α ΔT A

📐 Example from the infographic

Given:

• Carbon steel
• E = 200 GPa
• α = 12 × 10⁻⁶ /°C
• L = 10 m
• T₁ = 30°C
• T₂ = 180°C
• A = 1.5 × 10⁻³ m²
• Allowable stress = 120 MPa

Temperature difference:

ΔT = 180 − 30 = 150°C

Thermal stress:

σₜₕ = EαΔT

= (200 × 10⁹)(12 × 10⁻⁶)(150)

= 360 MPa

Thermal force:

Pₜₕ = σₜₕA

= (360 × 10⁶)(1.5 × 10⁻³)

= 540 kN

Since 360 MPa > 120 MPa, the calculated thermal stress exceeds the assumed allowable stress.

The corresponding maximum allowable temperature difference is:

ΔTₘₐₓ = Sₐₗₗₒw / (Eα)

= 50°C

27/08/2026

𝗦𝗧𝗢𝗥𝗔𝗚𝗘 𝗧𝗔𝗡𝗞 𝗗𝗘𝗦𝗜𝗚𝗡 𝗠𝗜𝗦𝗧𝗔𝗞𝗘𝗦 — #𝟬𝟭

𝗜𝗡𝗖𝗢𝗥𝗥𝗘𝗖𝗧 𝗧𝗔𝗡𝗞 𝗩𝗘𝗡𝗧𝗜𝗡𝗚 𝗗𝗘𝗦𝗜𝗚𝗡
𝗔 𝘀𝗺𝗮𝗹𝗹 𝗰𝗼𝗺𝗽𝗼𝗻𝗲𝗻𝘁. 𝗔 𝗯𝗶𝗴 𝗿𝗶𝘀𝗸.

A storage tank can have the correct shell thickness, foundation, roof structure and materials — yet still be vulnerable to serious damage if its 𝗩𝗘𝗡𝗧𝗜𝗡𝗚 𝗦𝗬𝗦𝗧𝗘𝗠 is incorrectly designed or sized.

One of the most common mistakes is treating venting as a simple valve-selection exercise.

𝗩𝗲𝗻𝘁𝗶𝗻𝗴 𝗱𝗲𝘀𝗶𝗴𝗻 𝘀𝘁𝗮𝗿𝘁𝘀 𝘄𝗶𝘁𝗵 𝗰𝗮𝗹𝗰𝘂𝗹𝗮𝘁𝗶𝗼𝗻 — 𝗻𝗼𝘁 𝘄𝗶𝘁𝗵 𝘃𝗮𝗹𝘃𝗲 𝘀𝗶𝘇𝗲.

According to the engineering framework of API 2000, several operating scenarios may need to be evaluated:

🔹 𝗡𝗢𝗥𝗠𝗔𝗟 𝗢𝗨𝗧𝗕𝗥𝗘𝗔𝗧𝗛𝗜𝗡𝗚
Liquid filling and thermal expansion can increase vapor-space pressure.

🔹 𝗡𝗢𝗥𝗠𝗔𝗟 𝗜𝗡𝗕𝗥𝗘𝗔𝗧𝗛𝗜𝗡𝗚
Liquid withdrawal and thermal contraction can create vacuum conditions.

🔹 𝗘𝗠𝗘𝗥𝗚𝗘𝗡𝗖𝗬 𝗩𝗘𝗡𝗧𝗜𝗡𝗚
External fire exposure can create venting demand far beyond normal operating conditions.

🔹 𝗣𝗥𝗘𝗦𝗦𝗨𝗥𝗘 / 𝗩𝗔𝗖𝗨𝗨𝗠 𝗟𝗜𝗠𝗜𝗧𝗦
Valve settings, pressure losses and accumulated pressure/vacuum must remain compatible with the tank's allowable limits.

🔹 𝗡𝗜𝗧𝗥𝗢𝗚𝗘𝗡 𝗕𝗟𝗔𝗡𝗞𝗘𝗧𝗜𝗡𝗚
A blanketing system does not replace properly designed pressure and vacuum protection.

🔹 𝗩𝗔𝗣𝗢𝗥 𝗥𝗘𝗖𝗢𝗩𝗘𝗥𝗬
VRU piping and equipment can introduce additional pressure drop and operating constraints that must be considered.

💡 𝗧𝗛𝗘 𝗞𝗘𝗬 𝗘𝗡𝗚𝗜𝗡𝗘𝗘𝗥𝗜𝗡𝗚 𝗣𝗥𝗜𝗡𝗖𝗜𝗣𝗟𝗘

𝗖𝗔𝗟𝗖𝗨𝗟𝗔𝗧𝗘 𝗧𝗛𝗘 𝗥𝗘𝗤𝗨𝗜𝗥𝗘𝗗 𝗩𝗘𝗡𝗧𝗜𝗡𝗚 𝗖𝗔𝗣𝗔𝗖𝗜𝗧𝗬 𝗙𝗜𝗥𝗦𝗧.
𝗦𝗘𝗟𝗘𝗖𝗧 𝗧𝗛𝗘 𝗗𝗘𝗩𝗜𝗖𝗘 𝗦𝗘𝗖𝗢𝗡𝗗.

The vent nozzle may be one of the smallest connections on a storage tank.

But getting its design wrong can create one of the largest operational risks.

💬 𝗘𝗡𝗚𝗜𝗡𝗘𝗘𝗥𝗜𝗡𝗚 𝗗𝗜𝗦𝗖𝗨𝗦𝗦𝗜𝗢𝗡:

What is the most common tank venting mistake you have encountered in design reviews, commissioning or field operation?

26/08/2026

Flow fraction in TEMA sheet:

When designing high-performance heat exchangers the flow fraction distribution is everything. A minor error in phase distribution or shell-side flow partitioning can lead to unexpected bypass, poor heat transfer coefficients, or even vibration issues.

Using HTRI to refine these parameters isn’t just about meeting specs. it’s about ensuring long-term thermal stability and operational reliability during the transition from design to commissionin!

flow fraction is important when you design shell and tube heat exchanger , the way that we can improve flow fraction in rating of S&T is according to second slide, what other experience do you have in S&T design and rating? what other parameters should check in design and how we should improve it?

26/08/2026

LMTD (Log mean temperature difference).!

Log Mean Temperature Difference (LMTD) is a fundamental concept in heat exchanger design and thermal analysis. It represents the effective average temperature driving force between the hot and cold fluids as they exchange heat through the heat-transfer surface.

Unlike a simple arithmetic average, LMTD accounts for the fact that the temperature difference between the two fluids changes continuously from the inlet to the outlet.

🔹 temperature differences

For a typical counter-current heat exchanger:

ΔT₁ = Tₕ,ᵢₙ − T𝑐,ₒᵤₜ

ΔT₂ = Tₕ,ₒᵤₜ − T𝑐,ᵢₙ

The LMTD is then calculated as:

ΔTₗₘ = (ΔT₁ − ΔT₂) / ln(ΔT₁ / ΔT₂)

This value provides the effective temperature driving force used in heat-transfer calculations.

🔹 heat-transfer calculation

Once LMTD is known, the heat-transfer rate can be estimated using:

Q = U × A × ΔTₗₘ

Where:

• Q = Heat-transfer rate (W)
• U = Overall heat-transfer coefficient (W/m²·K)
• A = Heat-transfer area (m²)
• ΔTₗₘ = Log mean temperature difference (K)

🔹 example from the infographic

Given:

• Hot-fluid inlet = 150°C
• Hot-fluid outlet = 90°C
• Cold-fluid inlet = 30°C
• Cold-fluid outlet = 60°C

Therefore:

ΔT₁ = 150 − 60 = 90 K

ΔT₂ = 90 − 30 = 60 K

So:

LMTD = (90 − 60) / ln(90/60) ≈ 74 K

With:

U = 800 W/m²·K
A = 50 m²

The heat-transfer rate is approximately:

Q = 800 × 50 × 74 = 2.96 MW

🔹 why is lmtd important?

LMTD is widely used when analyzing and designing:

✅ Shell-and-tube heat exchangers
✅ Plate heat exchangers
✅ Condensers
✅ Reboilers
✅ Evaporators
✅ Process heaters and coolers
✅ Waste-heat recovery systems

It helps engineers determine the required heat-transfer area, evaluate exchanger performance, and estimate the thermal duty for a given set of operating conditions.

📌 Engineering takeaway:
The larger the effective temperature driving force, the greater the potential heat-transfer rate for a given U and A. LMTD is therefore one of the key parameters connecting process temperatures with heat-exchanger sizing and performance.