diff --git a/OpenHPL/Data.mo b/OpenHPL/Data.mo
index 84a3f62..e04ede0 100644
--- a/OpenHPL/Data.mo
+++ b/OpenHPL/Data.mo
@@ -3,7 +3,7 @@ record Data "Provides a data set of most common used settings"
   extends Modelica.Icons.Record;
   parameter Boolean showElevation=true "Display elevation of connectors"
     annotation(Dialog(group = "Icon"),
-    choices(checkBox = true)); 
+    choices(checkBox = true));
   parameter SI.Acceleration g = Modelica.Constants.g_n "Gravity constant"
     annotation (Dialog(enable=false, group = "Constants"));
   parameter Real gamma_air = 1.4 "Ratio of heat capacities at constant pressure (C_p) to constant volume (C_v) for air at STP"
@@ -16,8 +16,24 @@ record Data "Provides a data set of most common used settings"
     annotation (Dialog(group = "Waterway properties"));
   parameter SI.DynamicViscosity mu = 1.3076e-3 "Dynamic viscosity of water at T_0"
     annotation (Dialog(group = "Waterway properties"));
-  parameter SI.Height p_eps = 0 "Pipe roughness height (default is smooth pipe)"
-    annotation (Dialog(group = "Waterway properties"));
+  parameter Types.FrictionMethod FrictionMethod = Types.FrictionMethod.PipeRoughness "Default friction specification method"
+    annotation (Dialog(group = "Friction"));
+  parameter SI.Height p_eps_input = 0 "Pipe roughness height (for PipeRoughness method)"
+    annotation (Dialog(group = "Friction", enable = FrictionMethod == Types.FrictionMethod.PipeRoughness));
+  parameter Real f_moody(min=0) = 0.02 "Moody friction factor (used when FrictionMethod = MoodyFriction)"
+    annotation (Dialog(group = "Friction", enable = FrictionMethod == Types.FrictionMethod.MoodyFriction));
+  parameter Real m_manning(unit="m(1/3)/s", min=0) = 40 "Manning M (Strickler) coefficient (used when FrictionMethod = ManningFriction)"
+    annotation (Dialog(group = "Friction", enable = FrictionMethod == Types.FrictionMethod.ManningFriction and not use_n));
+  parameter Boolean use_n = false "If true, use Manning's n instead of M"
+    annotation (Dialog(group = "Friction", enable = FrictionMethod == Types.FrictionMethod.ManningFriction), choices(checkBox=true));
+  parameter Real n_manning(unit="s/m(1/3)", min=0) = 0.025 "Manning's n coefficient (used when FrictionMethod = ManningFriction and use_n)"
+    annotation (Dialog(group = "Friction", enable = FrictionMethod == Types.FrictionMethod.ManningFriction and use_n));
+  parameter SI.Diameter D_h = 1.0 "Reference hydraulic diameter for friction conversion (used for Moody/Manning)"
+    annotation (Dialog(group = "Friction", enable = FrictionMethod <> Types.FrictionMethod.PipeRoughness));
+  final parameter Real n_eff_ = if use_n then n_manning else 1/m_manning "Effective Manning's n coefficient";
+  final parameter SI.Height p_eps = if FrictionMethod == Types.FrictionMethod.PipeRoughness then p_eps_input
+                                     elseif FrictionMethod == Types.FrictionMethod.MoodyFriction then 3.7 * D_h * 10^(-1/(2*sqrt(f_moody)))
+                                     else D_h * 3.0971 * exp(-0.118/n_eff_) "Computed equivalent pipe roughness height";
   parameter SI.Compressibility beta = 4.5e-10 "Water compressibility"
     annotation (Dialog(group = "Waterway properties"));
   parameter SI.Compressibility beta_total = 1 / (rho*1000^2) "Total compressibility"
diff --git a/OpenHPL/Types/FrictionSpec.mo b/OpenHPL/Types/FrictionSpec.mo
new file mode 100644
index 0000000..c816a05
--- /dev/null
+++ b/OpenHPL/Types/FrictionSpec.mo
@@ -0,0 +1,51 @@
+within OpenHPL.Types;
+partial model FrictionSpec "Reusable friction specification with multiple input methods"
+  outer Data data "Using standard data set";
+
+  parameter Types.FrictionMethod FrictionMethod = data.FrictionMethod "Method for specifying pipe friction" annotation (
+    Dialog(group = "Friction"));
+  parameter SI.Height p_eps_input = data.p_eps "Pipe roughness height (absolute)" annotation (
+    Dialog(group = "Friction", enable = FrictionMethod == Types.FrictionMethod.PipeRoughness));
+  parameter Real f_moody(min=0) = data.f_moody "Moody friction factor (dimensionless, typically 0.01-0.05)" annotation (
+    Dialog(group = "Friction", enable = FrictionMethod == Types.FrictionMethod.MoodyFriction));
+  parameter Real m_manning(unit="m(1/3)/s", min=0) = data.m_manning "Manning M (Strickler) coefficient M=1/n (typically 60-110 for steel, 30-60 for rock tunnels)" annotation (
+    Dialog(group = "Friction", enable = FrictionMethod == Types.FrictionMethod.ManningFriction and not use_n));
+  parameter Boolean use_n = data.use_n "If true, use Mannings coefficient n (=1/M) instead of Manning's M (Strickler)" annotation (
+    Dialog(group = "Friction", enable = FrictionMethod == Types.FrictionMethod.ManningFriction), choices(checkBox=true));
+  parameter Real n_manning(unit="s/m(1/3)", min=0) = data.n_manning "Manning's n coefficient (typically 0.009-0.017 for steel/concrete, 0.017-0.030 for rock tunnels)" annotation (
+    Dialog(group = "Friction", enable = FrictionMethod == Types.FrictionMethod.ManningFriction and use_n));
+
+  parameter SI.Diameter D_h "Hydraulic diameter used for friction conversion" annotation (
+    Dialog(group = "Friction"));
+
+protected
+  parameter Real n_eff = if use_n then n_manning else 1/m_manning "Effective Manning's n coefficient";
+  parameter SI.Height p_eps = if FrictionMethod == Types.FrictionMethod.PipeRoughness then p_eps_input
+                               elseif FrictionMethod == Types.FrictionMethod.MoodyFriction then 3.7 * D_h * 10^(-1/(2*sqrt(f_moody)))
+                               else D_h * 3.0971 * exp(-0.118/n_eff) "Equivalent pipe roughness height";
+
+  annotation (preferredView="info",
+    Documentation(info="<html>
+<h4>Friction Specification</h4>
+<p>Partial model providing a reusable friction parameter set. Extending models must supply
+the hydraulic diameter <code>D_h</code> used for converting Moody and Manning coefficients
+to equivalent pipe roughness height <code>p_eps</code>.</p>
+
+<p>Three friction specification methods are supported via the <code>FrictionMethod</code> parameter:</p>
+<ul>
+<li><strong>Pipe Roughness (p_eps)</strong>: Direct specification of absolute pipe roughness height (m).
+Typical values: 0.0001&ndash;0.001 m for steel pipes, 0.001&ndash;0.003 m for concrete.</li>
+<li><strong>Moody Friction Factor (f)</strong>: Dimensionless friction factor from Moody diagram.
+Typical values: 0.01&ndash;0.05. Converted to equivalent roughness using fully turbulent flow approximation:
+p_eps = 3.7&middot;D&middot;10<sup>-1/(2&radic;f)</sup></li>
+<li><strong>Manning Coefficient</strong>: Two notations are supported:
+<ul>
+<li><strong>Manning's M coefficient (Strickler)</strong> [m<sup>1/3</sup>/s]: M = 1/n, typical values 60&ndash;110 for steel,
+30&ndash;60 for rock tunnels.</li>
+<li><strong>Manning's n coefficient</strong> [s/m<sup>1/3</sup>]: Typical values 0.009&ndash;0.013 for smooth steel,
+0.012&ndash;0.017 for concrete, 0.017&ndash;0.030 for rock tunnels. Use checkbox <code>use_n</code> to enable.</li>
+</ul>
+Converted using: p_eps = D_h&middot;3.097&middot;e<sup>(-0.118/n)</sup> empirically derived from the Karman-Prandtl equation.</li>
+</ul>
+</html>"));
+end FrictionSpec;
diff --git a/OpenHPL/Types/package.order b/OpenHPL/Types/package.order
index d23f790..5071474 100644
--- a/OpenHPL/Types/package.order
+++ b/OpenHPL/Types/package.order
@@ -2,5 +2,6 @@ DraftTube
 Efficiency
 Fitting
 FrictionMethod
+FrictionSpec
 Lambda
 SurgeTank
diff --git a/OpenHPL/Waterway/DraftTube.mo b/OpenHPL/Waterway/DraftTube.mo
index 71ecde7..0847abe 100644
--- a/OpenHPL/Waterway/DraftTube.mo
+++ b/OpenHPL/Waterway/DraftTube.mo
@@ -2,6 +2,7 @@ within OpenHPL.Waterway;
 model DraftTube "Model of a draft tube for reaction turbines"
   outer Data data "Using standard data set";
   extends OpenHPL.Icons.DraftTube;
+  extends Types.FrictionSpec(   final D_h = 0.5 * (D_i + D_o));
   import Modelica.Constants.pi;
   parameter Types.DraftTube DraftTubeType = OpenHPL.Types.DraftTube.ConicalDiffuser "Types of draft tube" annotation (
     Dialog(group = "Draft tube types"));
@@ -29,8 +30,6 @@ model DraftTube "Model of a draft tube for reaction turbines"
              choice = 45 "45°",
              choice = 60 "60°",
              choice = 90 "90°"));
-  parameter SI.Height p_eps = data.p_eps "Pipe roughness height" annotation (
-    Dialog(group = "Geometry"));
   parameter Boolean SteadyState=data.SteadyState "If true, starts in steady state" annotation (Dialog(group="Initialization"));
   parameter SI.VolumeFlowRate Vdot_0=data.Vdot_0 "Initial volume flow rate" annotation (Dialog(group="Initialization"));
 
diff --git a/OpenHPL/Waterway/PenstockKP.mo b/OpenHPL/Waterway/PenstockKP.mo
index 31a592e..e1c68ae 100644
--- a/OpenHPL/Waterway/PenstockKP.mo
+++ b/OpenHPL/Waterway/PenstockKP.mo
@@ -2,6 +2,7 @@ within OpenHPL.Waterway;
 model PenstockKP "Detailed model of the pipe. Could have elastic walls and compressible water. KP scheme"
   outer OpenHPL.Data data "Using standard data set";
   extends OpenHPL.Icons.Pipe( vertical=true);
+  extends Types.FrictionSpec(   final D_h = (D_i + D_o) / 2);
   import Modelica.Constants.pi;
   // geometrical parameters of the pipe
   parameter SI.Height H = 420 "Height difference from the inlet to the outlet of the pipe" annotation (
@@ -12,8 +13,6 @@ model PenstockKP "Detailed model of the pipe. Could have elastic walls and compr
     Dialog(group = "Geometry"));
   parameter SI.Diameter D_o = D_i "Diametr from the outlet side of the pipe" annotation (
     Dialog(group = "Geometry"));
-  parameter SI.Height p_eps = data.p_eps "Pipe roughness height" annotation (
-    Dialog(group = "Geometry"));
   // condition of steady state
   parameter Boolean SteadyState=data.SteadyState "If true, starts in steady state" annotation (Dialog(group="Initialization"));
   // staedy state values for flow rate in all segments of the pipe
diff --git a/OpenHPL/Waterway/Pipe.mo b/OpenHPL/Waterway/Pipe.mo
index 3b88d2b..62f6b95 100644
--- a/OpenHPL/Waterway/Pipe.mo
+++ b/OpenHPL/Waterway/Pipe.mo
@@ -3,6 +3,7 @@ model Pipe "Model of a pipe"
   outer Data data "Using standard data set";
   extends OpenHPL.Icons.Pipe;
   extends OpenHPL.Interfaces.TwoContacts;
+  extends Types.FrictionSpec(   final D_h = (D_i + D_o) / 2);
 
   // Geometrical parameters of the pipe:
   parameter SI.Length H = 0 "Height difference from the inlet to the outlet" annotation (
@@ -14,21 +15,6 @@ model Pipe "Model of a pipe"
   parameter SI.Diameter D_o = D_i "Diameter of the outlet side" annotation (
     Dialog(group = "Geometry"));
 
-  // Friction specification:
-  parameter Types.FrictionMethod friction_method = Types.FrictionMethod.PipeRoughness "Method for specifying pipe friction" annotation (
-    Dialog(group = "Friction"));
-  parameter SI.Height p_eps_input = data.p_eps "Pipe roughness height (absolute)" annotation (
-    Dialog(group = "Friction", enable = friction_method == Types.FrictionMethod.PipeRoughness));
-  parameter Real f_moody(min=0) = 0.02 "Moody friction factor (dimensionless, typically 0.01-0.05)" annotation (
-    Dialog(group = "Friction", enable = friction_method == Types.FrictionMethod.MoodyFriction));
-  parameter Real m_manning(unit="m(1/3)/s", min=0) = 40 "Manning M (Strickler) coefficient M=1/n (typically 60-110 for steel, 30-60 for rock tunnels)" annotation (
-    Dialog(group = "Friction", enable = friction_method == Types.FrictionMethod.ManningFriction and not use_n));
-  parameter Boolean use_n = false "If true, use Mannings coefficient n (=1/M) instead of Manning's M (Strickler)" annotation (
-    Dialog(group = "Friction", enable = friction_method == Types.FrictionMethod.ManningFriction), choices(checkBox=true));
-  parameter Real n_manning(unit="s/m(1/3)", min=0) = 0.025 "Manning's n coefficient (typically 0.009-0.017 for steel/concrete, 0.017-0.030 for rock tunnels)" annotation (
-    Dialog(group = "Friction", enable = friction_method == Types.FrictionMethod.ManningFriction and use_n));
-
-
   // Steady state:
   parameter Boolean SteadyState=data.SteadyState "If true, starts in steady state" annotation (Dialog(group="Initialization"));
   parameter SI.VolumeFlowRate Vdot_0=data.Vdot_0 "Initial flow rate of the pipe" annotation (Dialog(group="Initialization"));
@@ -44,10 +30,6 @@ model Pipe "Model of a pipe"
   SI.VolumeFlowRate Vdot "Volume flow rate";
 
 protected
-    parameter Real n_eff = if use_n then n_manning else 1/m_manning "Effective Manning's n coefficient";
-    parameter SI.Height p_eps = if friction_method == Types.FrictionMethod.PipeRoughness then p_eps_input
-                                 elseif friction_method == Types.FrictionMethod.MoodyFriction then 3.7 * D_ * 10^(-1/(2*sqrt(f_moody)))
-                                 else D_*3.0971 *exp(-0.118/n_eff) "Equivalent pipe roughness height";
     parameter SI.Diameter D_ = ( D_i + D_o)  / 2 "Average diameter";
     parameter SI.Area A_i = D_i ^ 2 * C.pi / 4 "Inlet cross-sectional area";
     parameter SI.Area A_o = D_o ^ 2 * C.pi / 4 "Outlet cross-sectional area";
@@ -119,24 +101,8 @@ slopes (positive or negative height difference).</p>
 taper geometry: 0.05–0.15 for gentle cones, up to 0.6 for sharp contractions.</p>
 
 <h5>Friction Specification</h5>
-<p>The pipe friction can be specified using one of three methods via the <code>friction_method</code> parameter:</p>
-<ul>
-<li><strong>Pipe Roughness (p_eps)</strong>: Direct specification of absolute pipe roughness height (m).
-Typical values: 0.0001-0.001 m for steel pipes, 0.001-0.003 m for concrete.</li>
-<li><strong>Moody Friction Factor (f)</strong>: Dimensionless friction factor from Moody diagram.
-Typical values: 0.01-0.05. Converted to equivalent roughness using fully turbulent flow approximation:
-p_eps = 3.7·D·10<sup>-1/(2√f)</sup></li>
-<li><strong>Manning Coefficient</strong>: Two notations are supported:
-<ul>
-<li><strong>Manning's M coefficient (Strickler)</strong> [m<sup>1/3</sup>/s]: M = 1/n, typical values 60-110 for steel,
-30-60 for rock tunnels.</li>
-<li><strong>Manning's n coefficient</strong> [s/m<sup>1/3</sup>]: Typical values 0.009-0.013 for smooth steel,
-0.012-0.017 for concrete, 0.017-0.030 for rock tunnels.  Use checkbox <code>use_n</code> to enable this notation.</li>
-</ul>
-These are then converted using: p_eps = D_h·3.097·e<sup>(-0.118/n)</sup> empirically derived from the&nbsp;Karman-Prandtl equation.</li>
-</ul>
-<p>The conversions are simplified for hydropower applications assuming fully turbulent flow,
-so they depend only on fixed pipe dimensions and the chosen friction coefficient.</p>
+<p>Friction is specified via the inherited <a href=\"modelica://OpenHPL.Waterway.BaseClasses.FrictionSpec\">FrictionSpec</a>
+base class, which supports pipe roughness, Moody friction factor, and Manning coefficient methods.</p>
 
 <h5>Initialization</h5>
 <p>By default, the pipe provides an initial equation for the flow rate: either <code>der(mdot) = 0</code>
diff --git a/OpenHPL/Waterway/Reservoir.mo b/OpenHPL/Waterway/Reservoir.mo
index 951f082..afa4f22 100644
--- a/OpenHPL/Waterway/Reservoir.mo
+++ b/OpenHPL/Waterway/Reservoir.mo
@@ -1,4 +1,4 @@
-within OpenHPL.Waterway;
+within OpenHPL.Waterway;
 model Reservoir "Model of the reservoir"
   outer Data data "using standard class with constants";
   extends OpenHPL.Icons.Reservoir;
diff --git a/OpenHPL/Waterway/SurgeTank.mo b/OpenHPL/Waterway/SurgeTank.mo
index 687b775..e674200 100644
--- a/OpenHPL/Waterway/SurgeTank.mo
+++ b/OpenHPL/Waterway/SurgeTank.mo
@@ -3,6 +3,7 @@ model SurgeTank "Model of the surge tank/shaft"
   outer Data data "Using standard data set";
   extends OpenHPL.Icons.Surge(lds=l, Lds=L);
   extends OpenHPL.Interfaces.TwoContacts;
+  extends Types.FrictionSpec(   final D_h = D);
 
   parameter Types.SurgeTank SurgeTankType = OpenHPL.Types.SurgeTank.STSimple "Types of surge tank"
     annotation (Dialog(group = "Surge tank types"));
@@ -16,8 +17,6 @@ model SurgeTank "Model of the surge tank/shaft"
     annotation (Dialog(group = "Geometry"));
 
 
-  parameter SI.Height p_eps = data.p_eps "Pipe roughness height" annotation (
-    Dialog(group = "Geometry"));
   parameter SI.Diameter D_so = D "If Sharp orifice type: Diameter of sharp orifice" annotation (
     Dialog(group = "Geometry",enable=SurgeTankType == OpenHPL.Types.SurgeTank.STSharpOrifice));
   parameter SI.Diameter D_t = D "If Throttle value type: Diameter of throat" annotation (
@@ -113,9 +112,9 @@ equation
     F_f = Functions.DarcyFriction.Friction(v, D, l, data.rho, data.mu, p_eps) + A * phiSO * 0.5 * data.rho * abs(v) * v;
     F_p = (p_b - p_t) * A;
     if v >= 0 then
-      phiSO = Functions.Fitting.FittingPhi(v, D, D_so, L, 90, data.rho, data.mu, data.p_eps, OpenHPL.Types.Fitting.SharpOrifice);
+      phiSO = Functions.Fitting.FittingPhi(v, D, D_so, L, 90, data.rho, data.mu, p_eps, OpenHPL.Types.Fitting.SharpOrifice);
     else
-      phiSO = Functions.Fitting.FittingPhi(v, D_so, D, L, 90, data.rho, data.mu, data.p_eps, OpenHPL.Types.Fitting.SharpOrifice);
+      phiSO = Functions.Fitting.FittingPhi(v, D_so, D, L, 90, data.rho, data.mu, p_eps, OpenHPL.Types.Fitting.SharpOrifice);
     end if;
   elseif SurgeTankType == OpenHPL.Types.SurgeTank.STThrottleValve then
     if l <= L_t then
@@ -131,10 +130,10 @@ equation
       M = m * v;
       if v > 0 then
         F_f = Functions.DarcyFriction.Friction(Vdot/A_t, D_t, L_t, data.rho, data.mu, p_eps) + Functions.DarcyFriction.Friction(Vdot/A, D, l - L_t, data.rho, data.mu, p_eps) + A_t * phiSO * 0.5 * data.rho * abs(Vdot/A_t) * Vdot/A_t;
-        phiSO = Functions.Fitting.FittingPhi(Vdot/A_t, D_t, D, L, 90, data.rho, data.mu, data.p_eps, OpenHPL.Types.Fitting.Square);
+        phiSO = Functions.Fitting.FittingPhi(Vdot/A_t, D_t, D, L, 90, data.rho, data.mu, p_eps, OpenHPL.Types.Fitting.Square);
       elseif v < 0 then
         F_f = Functions.DarcyFriction.Friction(Vdot/A_t, D_t, L_t, data.rho, data.mu, p_eps) + Functions.DarcyFriction.Friction(Vdot/A, D, l - L_t, data.rho, data.mu, p_eps) + A * phiSO * 0.5 * data.rho * abs(Vdot/A) * Vdot/A;
-        phiSO = Functions.Fitting.FittingPhi(Vdot/A, D, D_t, L, 90, data.rho, data.mu, data.p_eps, OpenHPL.Types.Fitting.Square);
+        phiSO = Functions.Fitting.FittingPhi(Vdot/A, D, D_t, L, 90, data.rho, data.mu, p_eps, OpenHPL.Types.Fitting.Square);
       else
         F_f = 0;
         phiSO = 0;